Lighting devices with patterned printing of diffractive extraction features

ABSTRACT

Extended area lighting devices include a light guide and diffractive surface features on a major surface of the light guide, at least some diffractive surface features adapted to couple guided-mode light out of the light guide. The diffractive features include first and second diffractive features disposed on respective first and second portions of the major surface. A patterned light transmissive layer, including a second light transmissive medium, optically contacts the second diffractive features but not the first diffractive features. A first light transmissive medium optically contacts the first but not the second diffractive features. The first and second portions may define indicia, and the first and second diffractive features provide low distortion for viewing objects through the light guide such that the indicia is not readily apparent to users when guided-mode light does not propagate within the light guide. Optical films having such diffractive features are also disclosed.

FIELD OF THE INVENTION

This invention relates generally to lighting devices, with particularapplication to lighting devices that incorporate a light guide anddiffractive elements to couple guided-mode light out of the light guide.The invention also relates to associated articles, systems, and methods.

BACKGROUND

Extended area lighting devices that use a light guide to spread lightfrom discrete edge-mounted CCFL or LED light sources over the extendedarea of the light guide are known. Edge-lit backlights used in liquidcrystal displays (LCDs) are a major example of such lighting devices.Ordinarily, it is important for such lighting devices to have a colorand brightness that are uniform, or at least slowly varying, as afunction of position on the extended area output surface. It is alsoordinarily important for such lighting devices to emit light of asubstantially white color, so that the filtering action of the liquidcrystal panel can produce full color pixels and pictures ranging fromblue through red.

In order to extract guided-mode light out of the light guide, edge-litbacklights often configure a major surface of the light guide to have aprinted pattern of diffusive paint or other scattering material, or tohave a structured surface e.g. as provided by a series of grooves orprisms whose facets are designed to change the direction of light byrefraction or reflection. It is not common to extract guided-mode lightout of the light guide using diffractive grooves or prisms on the majorsurface, because diffraction has a strong wavelength dependence whichcould easily produce a highly colored appearance, and a highly coloredappearance is unacceptable in most end-use applications.

BRIEF SUMMARY

We have developed a new family of extended area lighting devices thatextract light from an extended light guide using diffractive surfacefeatures on a major surface of the light guide. Light from one or morelight sources is injected into the light guide, and at least some of thediffractive surface features interact with the injected light to coupleguided-mode light out of the light guide. The diffractive surfacefeatures include first and second diffractive surface features disposedon respective first and second portions of the major surface. Apatterned light transmissive layer, including a second lighttransmissive medium, optically contacts the second diffractive featuresbut not the first diffractive features. A first light transmissivemedium optically contacts the first but not the second diffractivefeatures. The first and second portions may define indicia which may bedecorative, utilitarian, or both. The first and second diffractivesurface features provide low distortion for non-guided mode light thatpropagates through the light guide, to permit viewing of objects throughthe light guide. The low distortion can be used to ensure that theindicia is not readily apparent to users when guided-mode light does notpropagate within the light guide. However, when the light sources areturned on or energized to provide the guided-mode light, the indicia canbecome readily apparent to users of the lighting device. The lightingdevice may thus provide logos or other indicia that are transparent orconcealed when the lighting device is in an “off” state, but that becomebright, illuminated, and revealed when the lighting device is in an “on”state. The lighting devices can be used as luminaires for generallighting or decorative lighting.

We have also developed optical films adapted for attachment tosubstrates to form light guides as summarized above. The optical filmsinclude a first major surface having the diffractive surface featuressummarized above, and a second major surface for attachment to asubstrate such as a clear plate.

We describe herein, inter alia, lighting devices that include a lightguide and a patterned light transmissive layer. The light guide has afirst major surface. First and second diffractive surface features areformed in respective first and second portions of the first majorsurface, and at least one of the first and second diffractive surfacefeatures are adapted to couple guided-mode light out of the light guide.The patterned light transmissive layer, which includes a second lighttransmissive medium, is in optical contact with the second diffractivesurface features but not the first diffractive surface features. Thelighting device also includes a first light transmissive medium inoptical contact with the first diffractive surface features but not thesecond diffractive surface features. The first and second lighttransmissive media may have different first and second refractiveindices respectively at a visible wavelength.

The first and second refractive indices may differ by at least 0.05, orby at least 0.1. The first and second portions of the first majorsurface may define indicia. The first and second refractive indices maybe sufficiently different so that differences in out-coupled lightbetween the first and second portions cause the indicia to be readilyapparent to a user of the lighting device when guided-mode lightpropagates within the light guide. The light guide may exhibit lowdistortion for viewing objects through the light guide in both the firstand second portions. The indicia may not be readily apparent to a userof the lighting device when guided-mode light does not propagate withinthe light guide.

The device may also include one or more light sources disposed proximatethe light guide to provide the guided-mode light in the light guide. Thefirst light transmissive medium may be air. The second lighttransmissive medium may be or comprise an adhesive. The first and secondlight transmissive media may both be polymer compositions. The first andsecond light transmissive media may both be substantially transparentand colorless.

The first and second diffractive surface features may have a diffractivesurface feature refractive index, the diffractive surface featurerefractive index differing from the first refractive index by a firstdifference dn1 and differing from the second refractive index by asecond difference dn2, the magnitude of dn2 being substantially lessthan that of dn1 such that the second diffractive surface featurescouple little or no guided-mode light out of light guide relative to thefirst diffractive surface features. In some cases, dn2 may have amagnitude less than half that of dn1 at a visible wavelength of light.Alternatively, dn1 and dn2 may have magnitudes comparable to each othersuch that substantial guided-mode light is coupled out of the lightguide by both the first and second diffractive surface features.

The lighting devices may also include a patterned low index subsurfacelayer configured to selectively block some guided mode light fromreaching the diffractive surface features. The patterned low indexsubsurface layer may in some cases include first and second layerportions, the first layer portion comprising nanovoided polymericmaterial, and the second layer portion comprising the nanovoidedpolymeric material and an additional material.

We also disclose optical films adapted for attachment to a substrate toform a light guide, such optical films including a first major surfacehaving first and second diffractive surface features formed in first andsecond portions respectively of the first major surface. At least one ofthe first and second diffractive surface features are adapted to coupleguided-mode light out of the light guide. The optical film also includesa patterned light transmissive layer in optical contact with the seconddiffractive surface features but not the first diffractive surfacefeatures, the patterned layer comprising a second light transmissivemedium. The optical film also includes a first light transmissive mediumin optical contact with the first diffractive surface features but notthe second diffractive surface features, and the first and second lighttransmissive media have different first and second refractive indicesrespectively at a visible wavelength.

The optical film may exhibit low distortion for viewing objects throughthe optical film in both the first and second portions. The first andsecond portions of the first major surface may define indicia. Theindicia may not be readily apparent to a user of the film before thefilm is attached to the substrate.

The film may further include a second major surface opposite the firstmajor surface, and a light transmissive adhesive layer disposed at thesecond major surface to facilitate attachment of the film to asubstrate. The film may also include a flexible carrier film and a prismlayer cast on the carrier film, and the first major surface of theoptical film may be an outer surface of the prism layer.

The disclosed lighting devices may also be adapted for use as securityarticles, e.g., articles intended for application to a product, package,or document as an indicator of authenticity, because the visual featuresare difficult to copy or counterfeit. Such articles thus preferablyinclude a light guide and a patterned light transmissive layer. Thelight guide has a first major surface. First and second diffractivesurface features are formed in respective first and second portions ofthe first major surface, and at least one of the first and seconddiffractive surface features are adapted to couple guided-mode light outof the light guide. The patterned light transmissive layer, whichincludes a second light transmissive medium, is in optical contact withthe second diffractive surface features but not the first diffractivesurface features. The lighting device also includes a first lighttransmissive medium in optical contact with the first diffractivesurface features but not the second diffractive surface features. Thefirst and second light transmissive media may have different first andsecond refractive indices respectively at a visible wavelength. Thelight guide may exhibit low distortion for viewing objects through thelight guide in both the first and second portions.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side or sectional view of a lighting deviceutilizing diffractive surface features on a major surface of a lightguide;

FIG. 2 is a schematic side or sectional view of a light guide with adiscrete light source injecting light into the light guide anddiffractive surface features coupling guided-mode light out of the lightguide;

FIG. 3 is a graph of intensity versus polar angle of integrated opticalpower density for light extracted from a light guide using lineardiffractive surface features;

FIG. 4 is a micrograph of a replicated diffractive surface useful forlight extraction from a light guide;

FIG. 5 is a conoscopic plot of measured intensity as a function of polarand azimuthal angle for a lighting device that used diffractive surfacestructures as shown in FIG. 4;

FIG. 5a is a graph of measured luminance versus polar angle along aparticular reference plane for the conoscopic plot of FIG. 5;

FIG. 6 is a schematic side or sectional view of a light guide havingasymmetric or blazed diffractive surface structures;

FIG. 7 is a graph of calculated extraction efficiency for the surfacestructures of FIG. 6;

FIG. 8 is a schematic side or sectional view of a lighting device thatincludes a plurality of light guides in a stacked or layeredarrangement;

FIG. 9 is a schematic perspective view of a lighting device thatincludes different diffractive surface features disposed on oppositemajor surfaces of the light guide and tailored for different coloredlight sources;

FIGS. 10 and 11 are schematic side or sectional views of light guideswith diffractive surface features, the diffractive surface featuresincluding groups of surface features of different pitches;

FIGS. 12-14 are schematic side or sectional views of lighting devices inwhich patterned printing is used to optically contact portions of themajor surface of a light guide with a patterned light transmissivelayer;

FIG. 15 is a schematic front or plan view of an embodiment in which thepatterned printing forms indicia in the shape of logos;

FIGS. 16 and 17 are schematic side or sectional views of additionallight guides with patterned printing, these light guides also having apatterned low index subsurface layer;

FIG. 16a is a schematic cross sectional view of an exemplary patternedlow index subsurface layer;

FIG. 18a is a photograph of a lighting device that was constructed usinga circular light guide having curved diffractive surface features andpatterned printing in the shape of a United States map in contact withdiffractive surface features, the lighting device photographedapproximately along an optical axis of the device and with ambient lighton and the discrete light sources of the lighting device turned off;

FIG. 18b is a photograph of the lighting device of FIG. 18a from aboutthe same viewing angle, but with ambient light off and the discretelight sources of the lighting device turned on;

FIG. 18c is a photograph of the lighting device of FIGS. 18a and 18b ,but at a more oblique viewing angle and from an opposite side of thelighting device;

FIG. 19 is a photograph of another lighting device having patternedprinting in the shape of a United States map in contact with diffractivesurface features, the lighting device photographed approximately alongan optical axis of the device and with ambient light off and thediscrete light sources of the lighting device turned on;

FIG. 20 is a photograph (with a magnified schematic inset) of a randomgradient dot pattern similar to one used to form a patterned low indexsubsurface layer for a lighting device;

FIG. 21a is a photograph of a lighting device having a rectangular lightguide, curved diffractive surface features, and a patterned low indexsubsurface layer (similar to that shown in FIG. 20), the device alsocapable of incorporating patterned printing in contact with thediffractive surface features; and

FIG. 21b is a photograph of the lighting device of FIG. 21a at anoblique viewing angle.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We have found that lighting devices such as luminaires can be made usingextended area light guides, diffractive surface features, and one ormore light sources, to provide general purpose illumination in a devicethat can also be aesthetically pleasing or otherwise have additionalfunctionality as a result of guided-mode light extraction by thediffractive surface features that is spatially non-uniform, i.e.,patterned, for a unique and interesting visual appearance. Thepatterning can be accomplished by applying, whether by selectiveprinting or by other suitable techniques, a light transmissive medium(which we may refer to as a second light transmissive medium) to makeoptical contact with some diffractive surface features (which we mayrefer to as second diffractive surface features) but not otherdiffractive surface features (which we may refer to as first diffractivesurface features) on a major surface of the light guide. The firstdiffractive surface features may instead be in optical contact with adifferent light transmissive medium (which we may refer to as a firstlight transmissive medium), whose refractive index at a visiblewavelength is different from that of the second light transmissivemedium. Depending on design details of the lighting device including thefirst and second light transmissive media, the patterning, and anyindicia associated with the patterning, may be substantially concealedor transparent when the lighting device is in an “off” state, butbright, illuminated, and revealed when the lighting device is in an “on”state with guided-mode light provided by the light source(s).

The lighting devices may also include other visual features such as oneor more bands or groups of bands having a 3-dimensional appearance, e.g.the bands may change shape as a function of viewing geometry (viewingposition and/or viewing angle), and/or multiple bands may form a patternhaving a 3-dimensional appearance for at least some viewing geometries.The change in shape is often associated with a change in curvature ofone or more of the bands, e.g., changing from straight to curved or viceversa, or from gently curved to more strongly curved or vice versa.

An exemplary lighting device 110 is shown in schematic side or sectionalview in FIG. 1. The lighting device 110 includes an extended area lightguide 112 and discrete light sources 114 a, 114 b. The lighting device110 may be mounted in any desired configuration but in this case it isshown mounted physically above the user 120, e.g. in or near a ceilingof a room or building. The device 110 may provide substantially whitelight illumination on a surface 122 such as a tabletop or floor.However, when the user 120 looks directly at the device 110, the usermay see a pattern of colors across the emitting area of the device 110.The user also desirably sees indicia or other spatial patterns in theemitting area of the device resulting from the patterned printingdiscussed elsewhere herein. The patterned printing provides a secondlight transmissive medium in optical contact with some diffractivesurface features on at least one major surface of the light guide. Otherdiffractive surface features on the same major surface are in opticalcontact with a different first light transmissive medium.

When looking directly at the device 110, the user may also see one ormore bands having a 3-dimensional appearance in the emitting area of thedevice. A given band is the result of the interaction of light emittedfrom one of the discrete light sources and diffractive surface featureson one or both major surfaces of the light guide. Alternatively, a givenband may be the result of the interaction of light reflected or absorbedby a localized region of high or low reflectivity in a non-uniformreflective structure extending along a side surface of the light guide.Details of such bands are described in commonly assigned patentapplication publication US 2014/0043846 (Yang et al.).

In addition to the pattern of colors, the indicia, and the bands in theemitting area of the device, the user 120 may also observe objects suchas object 124 through the light guide 112 with little or no opticaldistortion. Light emitted by or reflected by such objects is able topropagate through the light guide as non-guided-mode light, only a smallamount of which is deflected by the diffractive surface features.

The light guide 112 is extended along two in-plane directions, shown inFIG. 1 as an x- and y-axis of a Cartesian coordinate system, so that thelight guide has opposed major surfaces 112 a, 112 b, as well as sidesurfaces 112 c, 112 d. Diffractive surface features 113 are provided onat least one of the major surfaces of the light guide 112, such assurface 112 a as shown in the figure, or in other embodiments surface112 b, or both surfaces 112 a and 112 b. In any case, the diffractivesurface features are tailored to couple guided-mode light out of thelight guide by diffraction. The guided-mode light is shown in the figureas light 116, and out-coupled light emitted from the light guide isshown as light 117 a, 117 b. Light 117 a passes through the surface 112a in the general direction of the user 120 or surface 122, and light 117b passes through the surface 112 b in the general direction away fromthe user 120 or surface 122. In some cases the lighting device 110 maybe mounted so that the light 117 b provides indirect illumination to theroom, e.g. by redirecting the light 117 b back into the room byreflection from the ceiling or from another reflective member.

In this regard, a reflective film or layer may be applied to all or aportion of the surface 112 b, or it may be positioned near the surface112 b, so as to redirect the light 117 b so it emerges from the surface112 a. The reflective film may reflect light diffusely, specularly, orsemi-specularly, and may reflect light uniformly or non-uniformly as afunction of wavelength, and it may reflect normally incident lightuniformly or non-uniformly as a function of polarization. The reflectivefilm may for example be or comprise: white paint or paints of any othercolor; high reflectivity mirror films, e.g., films with metal coatingssuch as aluminum, silver, nickel, or the like, or non-metallic mirrorfilms such as 3M™ Vikuiti™ ESR; multilayer optical films having organic(e.g. polymeric) or inorganic constituent optical layers with a layerthickness profile tailored to reflect light over some or all of thevisible spectrum at normal incidence or at another desired incidenceangle; ESR films with diffuse coatings; white reflectors having glossysurfaces; reflectors with brushed metal surfaces, including films withmetal coatings whose surface is roughened to provide semi-specular ordiffuse reflectivity; reflectors with structured surfaces;microcavitated PET films; 3M™ Light Enhancement Films; and/or reflectivepolarizing films, including but not limited to Vikuiti™ DiffuseReflective Polarizer Film (DRPF), Vikuiti™ Dual Brightness EnhancementFilm (DBEF), Vikuiti™ Dual Brightness Enhancement Film II (DBEF II), andmultilayer optical films having different reflectivities for normallyincident light of different polarizations but an average reflectivity ofgreater than 50% for such normally incident light, over some or all ofthe visible spectrum. See also the optical films disclosed in: US2008/0037127 (Weber), “Wide Angle Mirror System”; US 2010/0165660 (Weberet al.), “Backlight and Display System Using Same”; US 2010/0238686(Weber et al.), “Recycling Backlights With Semi-Specular Components”; US2011/0222295 (Weber et al.), “Multilayer Optical Film with OutputConfinement in Both Polar and Azimuthal Directions and RelatedConstructions”; US 2011/0279997 (Weber et al.), “Reflective FilmCombinations with Output Confinement in Both Polar and AzimuthalDirections and Related Constructions”; WO 2008/144644 (Weber et al.),“Semi-Specular Components in Hollow Cavity Light Recycling Backlights”;and WO 2008/144656 (Weber et al.), “Light Recycling Hollow Cavity TypeDisplay Backlight”.

The light guide 112 may be physically thick or thin, but it ispreferably thick enough to support a large number of guided modes andfurthermore thick enough to efficiently couple to the emitting area ofthe discrete light sources. The light guide may, for example, have aphysical thickness in a range from 0.2 to 20 mm, or from 2 to 10 mm. Thethickness may be constant and uniform, or it may change as a function ofposition, as with a tapered or wedged light guide. If tapered, the lightguide may be tapered in only one in-plane direction, e.g. either the x-or the y-axis, or it may be tapered in both principal in-planedirections.

The light guide may be substantially flat or planar, ignoring smallamplitude surface variability associated with, e.g., diffractive surfacestructures. In some cases, however, the light guide may be non-flat,including simply curved, i.e., curved along only one principal in-planedirection, or complex curved, i.e., curved along both principal in-planedirections. The light guide may be entirely flat, entirely non-flat, orflat in some areas and non-flat in other areas. For light guides thatare non-flat along a particular in-plane direction, the cross-sectionalprofile along such a direction may be, for example, a simple arc, ormore complex non-straight contours. In some cases the light guide maydeviate greatly from a flat structure, e.g., the light guide may be inthe form of a solid or a hollow truncated hollow cone, wherein lightinjection can occur at the large end or the small end of the truncatedcone, as desired.

Whether or not the light guide 112 is flat, the light guide may have anouter boundary or edge whose shape, when the light guide is seen in planview, is curved, or piecewise flat (polygonal), or a combination ofpiecewise flat and curved. Examples of curved shapes are shapes withcontinuous arcs, such as circles, ovals, and ellipses, and shapes withdiscontinuous or undulating arcs, such as a sinusoid or sinusoid-likecontour. Examples of piecewise flat shapes are triangles, quadrilaterals(e.g., squares, rectangles, rhombuses, parallelograms, trapezoids),pentagons, hexagons, octagons, and so forth. The piecewise flat shapescan provide a straight or flat side surface or edge for light injectionfrom the discrete light sources, while curved shapes provide curved sidesurfaces for light injection.

The light guide is typically relatively rigid and self-supporting sothat it does not substantially bend or deform under its own weight, butflexible light guides can also be used and may, if desired, be held inplace using a support structure or frame, for example. The light guidemay have a unitary construction, or it may be made from a plurality ofcomponents attached to each other with no significant intervening airgaps, e.g., a thin structured surface film attached to a flat, smoothmajor surface of a thicker plate using a clear optical adhesive.

The light guide may be made of any suitable low loss light-transmissivematerial(s), such as glasses, plastics, or combinations thereof.Materials that are low loss, e.g., low absorption and low scatteringover visible wavelengths, are desirable so that guided-mode light canpropagate from one side surface completely across the light guide withabsorption/scattering losses that are small compared to losses due toout-coupling of such light by the diffractive surface features.Exemplary materials include suitable: glasses; acrylics; polycarbonates;polyurethanes; cyclo-olefin polymer/copolymers, including Zeonex™ andZeonor™ materials sold by Zeon Chemicals L.P, Louisville, Ky.; siliconesand elastomers; and pressure sensitive adhesives (PSAs) and otheradhesives, including silicone adhesives, 3M™ VHB™ conformable acrylicfoam tapes, and 3M™ OCA™ optically clear adhesives.

The device 110 also includes one or more discrete light sources 114 a,114 b, which are preferably mounted at an edge or side surface of thelight guide 112. The sources are discrete and small in size relative tothe in-plane dimension (length or width) of the light guide. However,light sources that are discrete or limited in size need not be used, andmay be replaced if desired with non-discrete light sources, includinglight sources whose emitting area is long and/or wide with respect tocorresponding dimensions of the side surface of the light guide. Thesources 114 a, 114 b are preferably solid state light sources such aslight emitting diodes (LEDs), but other suitable light sources can alsobe used.

In this regard, “light emitting diode” or “LED” refers to a diode thatemits light, whether visible, ultraviolet, or infrared, although in mostpractical embodiments the emitted light will have a peak wavelength inthe visible spectrum, e.g. from about 400 to 700 nm. The term LEDincludes incoherent encased or encapsulated semiconductor devicesmarketed as “LEDs”, whether of the conventional or super radiantvariety, as well as coherent semiconductor devices such as laser diodes,including but not limited to vertical cavity surface emitting lasers(VCSELs). An “LED die” is an LED in its most basic form, i.e., in theform of an individual component or chip made by semiconductor processingprocedures. For example, the LED die may be formed from a combination ofone or more Group III elements and of one or more Group V elements(III-V semiconductor). The component or chip can include electricalcontacts suitable for application of power to energize the device.Examples include wire bonding, tape automated bonding (TAB), orflip-chip bonding. The individual layers and other functional elementsof the component or chip are typically formed on the wafer scale, andthe finished wafer can then be diced into individual piece parts toyield a multiplicity of LED dies. The LED die may be configured forsurface mount, chip-on-board, or other known mounting configurations.Some packaged LEDs are made by forming a polymer encapsulant over an LEDdie and an associated reflector cup. Some packaged LEDs also include oneor more phosphor materials that are excited by an ultraviolet or shortwavelength visible LED die, and fluoresce at one or more wavelengths inthe visible spectrum. An “LED” for purposes of this application shouldalso be considered to include organic light emitting diodes, commonlyreferred to as OLEDs.

Light emitted by the sources such as sources 114 a, 114 b is injectedinto the light guide to provide guided-mode light, i.e., light that ispredominantly trapped in the light guide by total internal reflection(TIR), ignoring the effect of any diffractive surface features. Thelight emitted by each individual source is visible, and may be broadband (e.g. white) or narrow band (e.g. colored such as red, yellow,green, blue). If colored narrow band sources are used, different colorscan be combined to provide an overall white light illumination on thesurface 122, or the colors can be uniform, or different from each otherbut combined in such a way as to provide a decorative colored(non-white) illumination on the surface 122.

Diffractive surface features 113 are provided on at least one majorsurface of the light guide. These surface features or structures may beexposed to air, or planarized with a tangible material such as a low (orhigh) refractive index material, or both (some exposed to air, someplanarized) in a patterned arrangement. As discussed elsewhere herein,the diffractive surface features are sized and otherwise configured tocouple guided-mode light out of the light guide by diffraction, suchthat different wavelengths are coupled out differently, e.g. indifferent amounts, different directions, and different angulardistributions. The diffractive surface features may be tailored so thatlight from the edge-mounted light sources is emitted substantiallyequally from both major surfaces 112 a, 112 b of the light guide, orinstead so that the light is preferentially emitted from one of themajor surfaces, such as surface 112 a, which may then be designated theoutput surface of the light guide. In the latter case, the device may bemounted in a specific orientation so as to efficiently illuminate aroom, workspace, or other surface.

Although the diffractive surface features couple guided-mode light outof the light guide, the light guide and the diffractive surface featuresare preferably tailored so that non-guided-mode light, e.g., lightoriginating from a source or object behind the light guide and incidenton one of the major surfaces of the light guide, is minimally deviated(whether by diffraction or refraction) such that objects can be viewedthrough the light guide with low distortion. The low distortion mayprovide both aesthetic and utilitarian benefits. In FIG. 1, thedistortion is low enough so that the user 120 can view and recognize theobject 124 through the light guide 112. The object 124 may be theceiling or another neighboring structure which neither generates lightnor is part of the lighting device 110. Alternatively, the object 124may generate light and may be a part of the lighting device 110, forexample, it may be another edge-lit light guide with its own diffractivesurface features, or it may be a more conventional light source such asa spotlight or light bulb with no diffractive surface features butconnected to the light guide 112 and mounted such that most or at leastsome of the light it emits is directed through the light guide 112.Furthermore, the object 124 may be or include a graphic film disposednear or attached to the device 110.

The diffractive surface features 113 may be present over substantiallyall of the major surface 112 a, or only a portion of the surface. If thediffractive surface features cover only certain portions of the surface,light from the edge-mounted light sources may be emitted from the lightguide only in those portions.

Additional aspects of the diffractive surface features are discussedfurther below. One particularly noteworthy aspect is the patternedprinting (not shown in FIG. 1 but shown in other figures below) providedon some of the diffractive surface features in shapes and/or patternsthat are, at least when the device is turned on, visible to a user ofthe device as indicia, for example, when the user looks directly at thedevice.

In some cases, at least some of the diffractive surface features mayoptionally be non-straight in plan view, and light propagating withinthe light guide may interact with the diffractive surface features toproduce at least one band that intersects the non-straight diffractivesurface features. The band may be a bright band, or, in some cases, adark band. The band changes in appearance (e.g. shape) as a function ofthe viewing position of an observer 120 relative to the lighting device110. The non-straight diffractive features may be, for example, curvedor segmented in shape, or may have an undulating or disjointed shapecomprising curves and/or segments. However, in some cases, some or allof the diffractive surface features on one or both of the major surfacesof the light guide may be straight in plan view. Bright and/or darkbands can also be generated with the straight diffractive surfacefeatures when discrete light sources and/or discrete absorbers are used,but the shapes of such bands may not change curvature as a function ofviewing position.

The lighting device 110, and the other lighting devices disclosedherein, can be used as a luminaire or similar lighting device forgeneral illumination purposes or the like. The luminaire may be mountedin any desired position and orientation, e.g., on, within, or near aceiling of a room, or on, within, or near a wall of a room, or mountedon a post, stand, or other support structure. The luminaire may beoriented parallel to the ceiling, or parallel to the wall, or at anoblique or intermediate angle with respect to the ceiling or wall.

In FIG. 2, we see a schematic view of a light guide 212 with a lightsource such as a discrete light source 214 injecting light into thelight guide, and diffractive surface features 213 coupling guided-modelight 216 out of the light guide to provide out-coupled light 217 a, 217b. The light guide 212, which may be the same as or similar to the lightguide 112 discussed above, has a first major surface 212 a on which thediffractive surface features 213 are provided, a second major surface212 b opposite the first major surface, and a side surface 212 c throughwhich light from the light source 214 can enter the light guide. Thelight source 214 may include an active element 214 a such as one or moreLED dies which convert electricity into visible light, and one or morereflective members 214 b which help direct some of the misdirected lightfrom the element 214 a into the side surface 212 c of the light guide212. Guided-mode light 216 from the light source 214 propagates viatotal internal reflection (TIR) along and within the light guide 212over a range of angles α which may be measured relative to the localplane of the light guide, in this case, the x-y plane. Out-coupled light217 a, 217 b may be measured or characterized, at least in part, by thepolar angle θ between the direction of propagation of a given light ray217 c and an axis 217 d normal to the local plane of the light guide, inthis case, the z-axis. FIG. 2 also shows an incident light beam 218 aimpinging upon and entering the light guide 212 through the majorsurface 212 b, propagating through the light guide 212 asnon-guided-mode light, and exiting the light guide through the majorsurface 212 a as transmitted light beam 218 b. The transmitted beam 218b is preferably minimally deviated by the diffractive surface features213 such that objects can be viewed through the light guide 212 with lowdistortion.

We will now elaborate on relevant design characteristics of thediffractive surface features 213 that allow them to provide thefunctional properties discussed above. Typically, the diffractivesurface features 213 are grooves or ridges/prisms with well-definedfaces that follow predetermined paths in plan view. For purposes of FIG.2, we will assume for simplicity that the diffractive features 213follow straight, linear paths that are parallel to each other and to they-axis. This assumption is not as restrictive as it seems, because thestraight, linear features can approximate a very small portion orsection of diffractive surface features that follow curved paths in planview, such as concentric circles or spiral arcs. We also assume forsimplicity that the diffractive features 213 have a uniformcenter-to-center spacing known as “pitch”, which is labeled “p” in FIG.2. This assumption is also not as restrictive as it seems, because theuniformly spaced diffractive features 213 can approximate a very smallportion or section of diffractive surface features whose pitch p changesas a function of position. The diffractive surface features 213 are alsoassumed to have a depth (grooves) or height (prisms) “h” as shown inFIG. 2.

The diffractive surface features 213 with the assumed linearconfiguration and constant pitch can be referred to as a single-pitch(or periodic) one-dimensional (1D) diffraction grating. The single-pitch1D grating is directly coupled to, and forms the major surface 212 a of,the light guide 212, which we assume has a refractive index of n and isimmersed in air or vacuum. Light from the light source 214 of opticalwavelength λ is injected or launched into the light guide 212 throughthe side surface 212 c, and propagates primarily by TIR within and alongthe light guide as guided-mode light 216. When such light impinges uponand interacts with the diffractive surface features 213, a fraction (η)of the guided-mode light 216 is extracted as out-coupled light 217 a,217 b. The out-coupled or extracted light 217 a, 217 b propagates alonga direction that is orthogonal to the light guide surface (e.g. having apolar angle θ=0 in FIG. 2) when the following condition is met:m×(λ/n)=d×cos(α).  (1)In this equation: α refers to the angle at which the guided-mode lightimpinges on the grating surface, measured relative to the plane of thesurface substantially as shown in FIG. 2; m is the diffraction order; nis the refractive index of the light guide 212; λ is the wavelength oflight; and d is the grating pitch, which is labeled “p” in FIG. 2. Forexample, for green light with λ=530 nm launched on-axis (α=0 degrees)into an acrylic light guide having a refractive index n=1.5, the gratingpitch d (or p) should equal 353 nm, and only the first diffraction order(m=1) is possible. For other values of α and λ, the extraction directionwill in general no longer be orthogonal to the light guide surface.

A computer simulation can be used here to illustrate the angulardistribution characteristics of extracted or out-coupled light as afunction of the light source wavelength, for the single-pitch 1Ddiffraction grating. In order to fully characterize the angulardistribution, both polar angle (angle θ in FIG. 2) and azimuthal angle(the angle measured in the x-y plane relative to a fixed direction oraxis in the x-y plane) should be considered. For purposes of thesimulation, for simplicity, we assume: that the light source 214 and thelight guide 212 (including the diffractive surface features 213) extendinfinitely along axes parallel to the y-axis; that the pitch d (or p) is353 nm; and that the light source 214 has a Lambertian distribution inthe x-z plane, i.e., an intensity proportional to the cosine of α, forlight emitted by the light source 214 in air before impinging on theside surface 212 c. After running the simulation with these assumptions,we calculate the total integrated optical power density as a function ofthe polar angle θ for 3 different optical wavelengths λ, and plot theresults in FIG. 3. In that figure, curves 310, 312, 314 show theintegrated optical power density for the optical wavelengths λ of 450 nm(blue light), 530 nm (green light), and 620 nm (red light),respectively.

The simulated results of FIG. 3 demonstrate, among other things, thewavelength-dependent nature of light extraction using diffractivesurface features. Although the curves 310, 312, 314 overlap to someextent, their peak intensities occur at polar angles that differ fromeach other by more than 10 degrees, with the red and blue peaks beingseparated by almost 30 degrees.

In addition to the simulation, we also fabricated a single-pitch 1Ddiffraction grating to demonstrate its utility as a light extractor fora light guide. First, a diamond tip for a diamond turning machine (DTM)was shaped using a focused ion beam (FIB) to form a V-shaped diamond tipwith an included angle of 45 degrees. This diamond tip was then used tocut symmetric, equally spaced V-shaped grooves around the circumferenceof a copper roll to make a diffraction grating master tool. Acast-and-cure replication process was then used to transfer the gratingpattern from the master tool to a film substrate. A triacetate cellulose(TAC) film having a thickness of 3 mils (about 76 micrometers) was usedas a base film or substrate due to its low birefringence and itsrefractive index value (n=1.5), which matches well to the refractiveindex of typical light guide materials. This base film was applied tothe master tool with a thin acrylate resin coating therebetween. Theacrylate resin composition comprised acrylate monomers (75% by weightPHOTOMER 6210 available from Cognis and 25% by weight1,6-hexanedioldiacrylate available from Aldrich Chemical Co.) and aphotoinitiator (1% by weight Darocur 1173, Ciba Specialty Chemicals).Ultraviolet light from a mercury vapor lamp (“D” bulb) was used for bothcasting and post-curing the microreplicated resin on the base film. Thecasting roll temperature was set at 130 degrees F. (54 degrees C.), andthe nip pressure was set at 20 to 25 psi (about 138,000 to 172,000pascals).

A microphotograph of the structured or grooved surface of the resultingdiffraction grating film is shown in FIG. 4. The pitch of thediffractive surface features in this figure is about 400 nanometers, andthe depth of the grooves (or height of the prisms) is about 500nanometers.

This film was then laminated to a 2 mm thick acrylic plate, which wasclear, flat, and rectangular, using a layer of optically clear adhesive(3M™ Optically Clear Adhesive 8172 from 3M Company, St. Paul, Minn.)such that the diffraction grating faced away from the acrylic plate andwas exposed to air, and such that no significant air gaps were presentbetween the base film of the diffraction grating film and the flat majorsurface of the acrylic plate to which the film was adhered. Thelaminated construction thus formed a light guide having the single-pitch1D diffraction grating serving as diffractive surface features on onemajor surface of the light guide. The light guide included a flat,straight side surface extending parallel to the groove direction of thediffractive surface features, similar to the configuration of FIG. 2. Alight source was constructed using a linear array of orange-emittingLEDs (obtained from OSRAM Opto Semiconductors GmbH), each LED having acenter wavelength of about 590 nm and a full-width-at-half-maximum(FWHM) bandwidth of about 20 nm. The discrete character of theindividual LEDs was masked by placing a diffuser plate (type DR-50 fromAstra Products Inc., Baldwin, N.Y.) in front of the LEDs, i.e., betweenthe LEDs and the side surface of the light guide, to provideillumination that was more spatially uniform. The light source thusapproximated a linear light source emitting light that was approximatelymonochromatic at a wavelength of 590 nm.

The light source was energized, and the intensity of the out-coupledlight emitted through the diffractive surface features was measured as afunction of polar angle and azimuthal angle using a conoscopic camerasystem. The measured conoscopic intensity distribution is shown in FIG.5. In this figure, the direction of elongation of the light source, andthe groove direction, corresponds to azimuthal values of 0 and 180degrees. The measured intensity or luminance in an orthogonal referenceplane, i.e., in a plane corresponding to azimuthal values of 90 and 270degrees in FIG. 5, is plotted as a function of polar angle θ in FIG. 5a. The reader may note the similarity of the curve in FIG. 5a relative tothe shape of the curves 310, 312, 314 in FIG. 3. The reader may alsonote in reference to FIG. 5 that light is extracted by the 1Ddiffraction grating in a narrow crescent-shaped distribution that doesnot lie in a plane, but that shifts in azimuthal angle as a function ofpolar angle.

Other aspects of the extended area lighting device discussed inconnection with FIGS. 4, 5, and 5 a include: light is extracted orout-coupled equally from both major surfaces of the light guide (seee.g. surfaces 212 a, 212 b of FIG. 2), which is a result of thesymmetric design of the diffractive surface features (i.e., thesymmetric V-shaped grooves that form the linear diffraction grating); ifthe monochromatic source is replaced with a white light source and/ormulti-colored light sources, angular color separation will occur as aresult of the diffraction phenomenon (see e.g. FIG. 3); no diffusercomponent is needed in the device (although in the embodiment of FIGS. 5and 5 a one is included in the light source to mask the discrete natureof the LED light sources) due to the fact that TIR is relied upon toallow the guided-mode light to propagate along the waveguide, anddiffraction is relied upon to extract or out-couple the light from thelight guide; and the crescent-shaped distribution of out-coupled lightis characterized by a relatively narrow light extraction angle.

Guided-mode light may be extracted or out-coupled preferentially throughone major surface of the light guide rather than the other major surfaceby changing the shape of the diffractive surface features, inparticular, making the shape of the individual features (e.g. prisms)asymmetrical. We demonstrate this in connection with FIGS. 6 and 7. InFIG. 6, a lighting device 610 includes a light guide 612 having a firstmajor surface 612 a and an opposed second major surface 612 b. The firstmajor surface 612 a comprises diffractive surface features 613 in theform of facets which form right-angle prism structures of height “h” andpitch “p”. The device 610 also includes a light source 614 disposedproximate a side surface of the light guide 612 to inject light into thelight guide as guided-mode light, such light propagating generally fromleft to right from the perspective of FIG. 6. A computer simulation ofthe device 610 was performed. In the simulation, for simplicity, theprism structures of the diffractive surface features 613 were assumed tobe equally spaced, and extending linearly along axes parallel to they-axis. The light source was also assumed to extend linearly parallel tothe y-axis, and was assumed to emit polarized light of wavelength λ intoair in a Lambertian distribution in a first reference plane parallel tothe plane of the light guide (see the x-y plane in FIG. 2), this lightthen being refracted at the side surface of the light guide. Thesimulation assumed only one propagation angle of light, α=5 degrees asreferenced in FIG. 2, in a second reference plane (see the x-z plane inFIG. 2) perpendicular to the first reference plane. The refractive indexof the light guide was assumed to be 1.5. The optical wavelength λ andthe grating pitch p were initially selected such that the out-coupledlight was extracted orthogonal to the light guide surface for firstorder diffraction (m=1), which yielded λ≈520 nm and p≈350 mm. Thegrating height h was then varied over a range from 50 to 500 nm, whilethe pitch p was held constant at 350 nm. For each embodiment associatedwith a specific value for the grating height, the following quantitieswere calculated by the computer simulation software:

-   -   extraction efficiency for transverse magnetic (TM) polarized        light extracted from the first major surface 612 a, referred to        here as TM-top extraction efficiency;    -   extraction efficiency for transverse electric (TE) polarized        light extracted from the first major surface 612 a, referred to        here as TE-top extraction efficiency;    -   extraction efficiency for transverse magnetic (TM) polarized        light extracted from the second major surface 612 b, referred to        here as TM-bottom extraction efficiency; and    -   extraction efficiency for transverse electric (TE) polarized        light extracted from the second major surface 612 b, referred to        here as TE-bottom extraction efficiency.        In this regard, “extraction efficiency” refers to the amount        (expressed as a percentage) of specified light (TM or TE)        extracted from the specified major surface (612 a or 612 b) for        a single interaction, divided by the amount of such specified        light propagating within the light guide immediately before the        interaction of the light beam with the extraction surface.

The calculated quantities are plotted in FIG. 7, where curve 710 is theTM-bottom extraction efficiency, curve 712 is the TE-bottom extractionefficiency, curve 714 is the TM-top extraction efficiency, and curve 716is the TE-top extraction efficiency. These results demonstrate thatguided-mode light can be extracted preferentially through one majorsurface of the light guide by making the shape of the individualdiffractive features (e.g. prisms) asymmetrical. The results alsodemonstrate that the degree to which light is preferentially extractedfrom one major surface depends on details of the particular shape of thediffractive features. In the case of right-angle prism features,preferential extraction can be maximized by selecting a height happroximately equal to the pitch p.

The diffractive surface features may be tailored so that light emittedfrom one major surface of the light guide (e.g. out-coupled light 217 ain FIG. 2) is the same as, or similar to, the light emitted from theopposed major surface of the light guide (e.g. out-coupled light 217 bin FIG. 2). The light emitted from the opposed surfaces may be the samewith respect to color, intensity, and/or the angular distribution ofcolor and/or intensity of the out-coupled light. In one approach,diffractive surface features may be provided on both opposed majorsurfaces, and these diffractive surface features may be mirror images ofeach other with respect to a reference plane disposed between andequidistant from the opposed major surfaces, such that the lightingdevice possesses mirror image symmetry with respect to such a referenceplane. In alternative embodiments, the diffractive surface features maybe tailored so that light emitted from one major surface of the lightguide is substantially different from the light emitted from the opposedmajor surface of the light guide. The light emitted from the opposedsurfaces may be different with respect to color, intensity, and/or theangular distribution of color and/or intensity of the out-coupled light.For example, an observer may perceive that light of one color is emittedfrom one major surface, and light of a substantially different color isemitted from the opposed major surface. In a horizontally-mountedlighting device, white light sources may be used with suitably tailoreddiffractive surface features such that white light of a relatively coolcolor temperature (bluish tint) is directed upwards towards the ceiling,and white light of a relatively warmer color temperature (reddish tint)is directed downwards towards the floor, or vice versa.

In applications where the angular separation of different colors oflight due to diffraction is undesirable, several design approaches canbe used to overcome the color separation issue. In one approach, shownin FIG. 8, two or more light guides can be stacked together. In anotherapproach, shown in FIG. 9, different diffractive surface features aredisposed on opposite major surfaces of a given light guide, and tailoredfor different colored light sources. In still another approach, shown inFIGS. 10 and 11, the diffractive surface features on a given majorsurface of a light guide may include groups of surface features ofdifferent pitches. Note that although these approaches are presented inconnection with dealing with the color separation issue, they may alsobe used for other purposes including utilitarian and/or aestheticpurposes in which color separation still occurs, or in single-colorembodiments that employ only light sources of a given desired(non-white) color. Note also that although the various approaches aredescribed individually, any two or more of the approaches can becombined together and used in a single embodiment.

Turning then to FIG. 8, we see there a schematic view of a lightingdevice 810 that includes a plurality of light guides 812, 832, 852 in astacked or layered arrangement. Each light guide has a pair of opposedmajor surfaces, i.e., light guide 812 has major surfaces 812 a, 812 b,light guide 832 has major surfaces 832 a, 832 b, and light guide 852 hasmajor surfaces 852 a, 852 b. At least one major surface of each lightguide preferably includes diffractive surface features, for example,major surface 812 a may include diffractive surface features 813, majorsurface 832 a may include diffractive surface features 833, and majorsurface 852 a may include diffractive surface features 853. The device810 also includes light sources 814 a, 814 b, 834 a, 834 b, 854 a, 854 barranged as shown to inject light into the respective light guides e.g.through their respective side surfaces, so as to provide guided-modelight in the light guides. Preferably, each of the light guides(including their diffractive surface features) has a low opticaldistortion such that non-guided-mode light can pass through the lightguide relatively undisturbed. In this way, light extracted from thelight guide 832 by the diffractive surface features 833 can pass throughthe light guide 812 to reach a user 820 and/or surface 822, and lightextracted from the light guide 852 by the diffractive surface features853 can pass through both light guide 812 and light guide 832 to reachthe user 820 and/or surface 822. Furthermore, the user 820 may alsoobserve objects such as object 824, which may be the same as or similarto object 124 discussed above, through the stack of light guides 812,832, 852 with little or no optical distortion.

If it is desirable to overcome the color separation issue, the variouslight guides, light sources, and diffractive surface features in thedevice 810 may be tailored to provide different colors of out-coupledlight to the user 820 and/or surface 822 so that the sum of all suchlight provides substantially white light illumination. For example, thelight sources 854 a, 854 b may emit red light and the diffractivesurface features 853 may optimally extract such light along an opticalaxis (e.g. an axis parallel to the z-axis) of the device, and the lightsources 834 a, 834 b may emit green light and the diffractive surfacefeatures 833 may optimally extract the green light along the sameoptical axis, and the light sources 814 a, 814 b may emit blue light andthe diffractive surface features 813 may optimally extract the bluelight along the same optical axis. Of course, red, green, and blue inthe order described are merely examples, and the reader will understandthat a multitude of alternative combinations are contemplated.Furthermore, although three light guides are shown in the stack of FIG.8, other numbers of light guides, including two, four, or more, can alsobe used. The constituent components of each layer within the stack mayall have the same or similar design, e.g., the same light guidedimensions and characteristics, the same dimensions and characteristicsof the diffractive surface structures, and the same numbers, colors, andarrangements of LEDs. Alternatively, the constituent components of eachlayer may differ from corresponding components in other layers in any ofthese respects. Similar to lighting device 110, the device 810 mayprovide substantially white light illumination on the surface 822, whileproviding a colored appearance when the user 820 looks directly at thedevice 810. Also, the user desirably sees spatial pattern(s) such asindicia in the emitting area of the device 810, which pattern(s) orindicia may originate with any one, or some, or all of the layers withinthe stack.

Turning to FIG. 9, we see there a schematic view of a lighting device910 that includes a light guide 912, and light sources 914 a, 914 bdisposed to inject light into different (e.g. orthogonal) side surfacesof the light guide. The light guide 912 has a pair of opposed majorsurfaces 912 a, 912 b. In device 910, each major surface has its owndiffractive surface features: surface 912 a has diffractive surfacefeatures 913 a, and surface 912 b has diffractive surface features 913b. The diffractive surface features are represented only schematicallyin the figure, but indicate that features 913 a extend generallyparallel to one in-plane axis (e.g. the y-axis), and the features 913 bextend generally parallel to an orthogonal in-plane axis (e.g. thex-axis). The light sources are likewise positioned and configured toinject light generally along orthogonal in-plane directions, with source914 a disposed to inject light generally along the x-axis and source 914b disposed to inject light generally along the y-axis. The term“generally” is used here because the light sources need not be (and inmany cases are not) collimated, but emit light in a distribution ofangles in the x-y plane. Also, although the sources 914 a, 914 b areeach shown as a discrete point source such as a single LED emitter, theymay alternatively each be a linear array of such discrete sourcesextending along the respective side surface of the light guide, or alinear or bar-shaped extended source. Nevertheless, light from thesource 914 a propagates predominantly along the in-plane x-axis, suchthat it interacts strongly with the diffractive surface features 913 aand weakly with the diffractive surface features 913 b, and light fromthe source 914 b propagates predominantly along the in-plane y-axis,such that it interacts weakly with the features 913 a and strongly withthe features 913 b.

This selective coupling of the light sources to different respectivediffractive surface features on the light guide using geometry ordirectionality can, if desired, be used to address the color separationissue. For example, the light sources may be substantially complementaryin their emission spectra, e.g., source 914 a may emit blue light andsource 914 b may emit yellow light, in which case the diffractivesurface features 913 a may be configured to extract blue light along agiven direction such as an optical axis (e.g. the positive z-axis) ofthe lighting device 910, while the diffractive surface features 913 bmay be configured to extract yellow light along the same direction, soas to provide substantially white light illumination along the opticalaxis. There is little interaction between the blue or yellow light withthe diffractive surface features (light extraction grating) of theopposite color because, as explained above, the grooves for blue lightextraction extend generally along the light path of the yellow light,the grooves for yellow light extraction extend generally along the lightpath of the blue light. The different colored light beams are thusguided and extracted independently in the same light guide. The combinedvisual effect of the out-coupled blue and yellow light gives rise to asensation of white light to an observer or user. The color renderingindex (CRI) of the white light in this example may however be relativelylow, because the light guide 912 combines only two colors.

The approach shown in FIG. 9 can be extended to numerous otherembodiments, including embodiments that use light sources of othercolors, including combinations of different complementary colors, andcolors that are not complementary, including also colors that may be thesame (e.g. green-emitting light for both sources 914 a and 914 b, orred-emitting light for both sources). Also, a lighting device such asdevice 910 can be combined with other lighting devices of similar ordifferent design, e.g. in a stacked arrangement as described inconnection with FIG. 8. In such a case, each light guide may beconfigured to emit a combination of two distinct colors, and the colorscollectively emitted from the stack may be selected to produce whitelight with a higher CRI, if desired.

Another approach that may be used to address the color separation issueis the approach shown generally in FIGS. 10 and 11. In these figures,light guides 1012, 1112 are shown in which the diffractive surfacefeatures on a given major surface include groups or packets of surfacefeatures of different pitches. The multiple different pitches can beused generally to provide a desired distribution of various wavelengthsof extracted light from the light guide, assuming light of suchwavelengths is injected into the light guide by one or more lightsources (not shown).

As mentioned elsewhere, the light guides disclosed herein may have avariety of different constructions, including a unitary construction, ora layered construction in which two or more components are attached toeach other with no significant intervening air gaps. In this regard, thelight guides 1012, 1112 are shown to have layered constructions, butthey may be readily modified to have a unitary construction if desired.Conversely, light guides shown as being unitary in other figures may bereadily modified to have layered constructions. In reference to FIG. 10,the light guide 1012 includes a relatively thick plate or othersubstrate 1011 a, to which is attached a film made up of a carrier film1011 b on which a prism layer 1011 c has been cast and cured. Thesubstrate 1011 a, carrier film 1011 b, and prism layer 1011 c preferablyhave the same or similar index of refraction, and are preferably allhighly transmissive to visible light, with little or no scattering orabsorption, although in some cases a controlled amount of absorptionand/or scattering may be acceptable or even desirable. In reference toFIG. 11, the light guide 1112 may have a similar construction to lightguide 1012, and thus may include a relatively thick plate or othersubstrate 1111 a, to which is attached a film made up of a carrier film1111 b on which a prism layer 1111 c has been cast and cured.

Attachment of a prismatic or structured surface film to a plate or othersubstrate to provide a layered light guide can be done by any suitabletechnique. For example, attachment can be achieved using a suitableadhesive, such as a light-transmissive pressure sensitive adhesive.Attachment may also be achieved using injection molding processes,including insert injection molding processes. Chemical bonds can also beused for attachment, e.g., when a curable resin is cast and cured on asuitable substrate such as a carrier film. Alternatively, in the case ofunitary constructions, the diffractive surface features can be formed onat least one surface of a unitary substrate such as a film or plate,e.g. by embossing or molding, including for example injection moldingprocesses. Compression molding, extrusion replication, and directcutting are additional techniques that may be used to form thediffractive surface features. Regardless of whether the diffractivestructures are formed on the surface of a film, plate, or othersubstrate, the diffractive surface features may be fabricated using anysuitable technique now known or later developed. Additional methods thatcan be used to make suitable diffractive surface features are discussedin one or more of: WO 2011/088161 (Wolk et al.); US 2012/0098421(Thompson); and US 2012/0099323 (Thompson).

The light guides 1012, 1112 have respective first major surfaces 1012 a,1112 a, and respective second major surfaces 1012 b, 1112 b opposite thefirst surfaces, as well as side surfaces (not shown). Similar to otherlight guides described herein, the first major surfaces 1012 a, 1112 aare configured to have diffractive surface features 1013, 1113,respectively. The surface features may be referred to as grooves orprisms. The grooves/prisms are shown as having an asymmetric 90 degreesawtooth profile in cross section, but other profiles can also be usedas desired including other asymmetric profiles and symmetric (e.g.V-shaped) profiles. In plan view the grooves/prisms may follow pathsthat are straight, curved, or both (e.g. straight in some places andcurved in other places). Significantly, the diffractive surface features1013, 1113 are arranged into groups or packets, the prisms or grooves inany given packet having a uniform pitch but adjacent packets havingdifferent pitches. In some cases, the packets can be arranged inpatterns that repeat across the surface of the light guide, the smallestrepeating group of packets being referred to here as a “set” of packets.For example, light guide 1012 (FIG. 10) has diffractive surface features1013 which is divided into groove or prism packets 1030, 1031, and 1032,these packets being arranged in a repeating sequence which defines sets1040. The prisms or grooves in each of packets 1030, 1031, 1032 have auniform pitch, but the pitch in packet 1030 is less than that in packet1031, which in turn is less than that in packet 1032. Light guide 1112(FIG. 11) has diffractive surface features 1113 which is divided intogroove or prism packets 1130, 1131, 1132, 1133, 1134, and 1135. Thesepackets may also be arranged in a repeating sequence to define set 1140.The prisms or grooves in each of packets 1130, 1131, 1132, 1133, 1134,and 1135 have a uniform pitch, but the pitch gets progressively largeras one moves from packet 1130 to packet 1135. Note that althoughdifferent pitches are used in the various packets shown in FIGS. 10 and11, preferably every one of the pitches is in a range suitable forcoupling some visible guided-mode light out of the light guide byprinciples of diffraction.

The width (in-plane transverse dimension) of the packets and the widthof the sets of packets, when the light guide is seen in plan view, maybe small enough so that they are visually imperceptible to the ordinaryobserver. Alternatively, the width of the packets and/or the widths ofthe sets of packets may be large enough so that they are perceptible asindicia or as an aesthetic pattern to the ordinary observer. If thepackets are designed to form perceptible indicia, such pitch-relatedindicia may in some cases be made to be in registration with indiciaformed by the patterned printing on the diffractive surface features,while in other cases the pitch-related indicia may not be inregistration with the indicia formed by the patterned printing, e.g.,the pitch-related indicia may partially overlap the patterned printingindicia, or there may be no overlap between the pitch-related indiciaand the patterned printing indicia.

Multiple pitch extraction designs such as those depicted in FIGS. 10 and11 can be used for color mixing. Generally speaking, at least twodifferent packets, characterized by two different pitches, can be used,but in many cases at least three different packets, characterized bythree different pitches p1, p2, p3, are desirable. The choice of thepitch dimension is a function of the refractive index (n) of the lightguide, as well as a function of the wavelength of light (λ) we wish toextract from the light guide with the given packet. In an exemplary casewe may select p1=λ1/n, where λ1 is in a range from 400 to 600 nm, andp2=λ2/n, where λ2 is in a range from 500 to 700 nm, and p3=λ3/n, whereλ3 is in a range from 600 to 900 nm. In the case of light guides made ofacrylic (n≈1.49) or similar materials, these conditions correspond to apitch p1 in a range from about 268 to 403 nm, p2 in a range from about336 to 370 nm, and p3 in a range from 403 to 604 nm. Polychromatic lightsuch as white light propagating within the light guide interacts withthe multiple pitch packets so that light of different colors isdiffracted (out-coupled or extracted from the waveguide) at differentangles for each given packet, the extraction angle for any given coloralso being different for the different packets. As a result, light ofthe various colors can be mixed or combined to provide illumination withsubstantial color uniformity, e.g. substantially white light, for usersor objects disposed at a suitable distance from the light guide.

In exemplary embodiments, the lighting device may utilize a plurality oflight sources having different spectral outputs, and a controller can beused to independently control the different light sources to actively ordynamically control the perceived color of the light emitted by thelighting device. This active control can be used to adjust or otherwisechange the color temperature, correlated color temperature, and/or thecolor rendering index (CRI) of the output light. Assemblies orcombinations of red, green, and blue-emitting LEDs (RGB), or red, green,blue, and white-emitting LEDs (RGBW), are of particular benefit for thispurpose. Also, light guides that incorporate a multiple pitch extractiondesign are also of particular benefit. Preferably, the multiple pitchdesign incorporates at least one packet of diffractive features of agiven pitch for each narrow-band emitting light source, e.g., one ormore packets whose pitch is tailored for red light, one or more packetswhose pitch is tailored for green light, one or more packets whose pitchis tailored for blue light, and so forth. Note that individual narrowband colors are not limited to red, green, and blue, and light sourcesthat emit other non-white colors such as yellow or amber may also beused to expand the color gamut of the disclosed lighting devices.

A design parameter of interest for the multi-pitch grating design, aswell as for other disclosed diffractive surface feature designs, is theeffective extraction efficiency. Extraction efficiency was discussedabove and will not be repeated here. The “effective” extractionefficiency refers to the percentage of specified light extracted fromthe specified major surface (612 a or 612 b) upon a single interaction,divided by the amount of such specified light propagating within thelight guide immediately before the interaction with the extractionsurface. The effective extraction efficiency for diffractive surfacefeatures (grooves or prisms) of a given pitch can be evaluated andcompared to the effective extraction efficiencies of other pitches. Ingeneral with given system parameters, the effective extractionefficiency of a given pitch: is a linear function of (i.e., directlyproportional to) the plan-view area coverage of diffractive featureshaving that pitch (e.g., for the smallest pitch in FIG. 10, the sum ofthe plan-view areas of the three packets 1030 on the surface); and alsodepends on other factors including the pitch of the diffractive featuresand the cross-sectional profile shape of the diffractive features(grooves/prisms). In order to obtain substantial color uniformity, it isdesirable to ensure that the effective extraction efficiencies for thedifferent pitches are comparable to each other, e.g., the ratio ofeffective extraction efficiencies for any two distinct pitchespreferably lies within the range from about 0.3 to 3.

As we saw in connection with FIGS. 4, 5, and 5 a, a monochromaticLambertian light source used to inject light into a light guide having asingle pitch linear diffraction grating gives rise to a crescent-shapeddistribution of out-coupled light characterized by a relatively narrowlight extraction angle. If even further angular narrowing of theout-coupled light is desired, the light source can be reconfigured withsuitable lenses, mirrors, or other components to emit light that iscollimated or nearly collimated rather than Lambertian. Conversely, ifangular widening of the out-coupled light is desired, the light sourcecan be reconfigured to emit light over a broader angular range than aLambertian distribution. Microstructured optical films can be combinedwith light sources such as LEDs or lasers to tailor the angular spreadof light injected into the light guide, thereby also affecting theangular spread of the out-coupled light. Suitable microstructuredoptical films are described in PCT Patent Publications WO 2012/075352(Thompson et al.) and WO 2012/075384 (Thompson et al.). These opticalfilms, which may be referred to as uniformity tapes, are applieddirectly to the edge or side surface of a light guide and compriserefractive structures facing outward toward the light source to enhancecoupling of light into the light guide. The refractive structures mayalternatively be incorporated directly into the side surface orinjection edge of the light guide, e.g. by injection molding, embossing,or direct machining. Such optical films or refractive structures, whendisposed between an LED source and the side surface of a light guide,can broaden the angular spread of light injected into the light guide,and can be used with one, some, or all of the light sources in any ofthe embodiments disclosed herein. Optical films with custom designedreplicated structures can also be used with coherent lasers to provide awell-defined rectangular-shaped angular distribution of light (i.e., alight distribution of approximately constant intensity over a specifiedcone of angles, and zero or near zero intensity outside the specifiedcone) for injection into the light guide.

The angular spread of the out-coupled light can also be tailored byappropriate selection of the physical width (in-plane transversedimension) of the packets of diffractive features, where the physicalwidth is measured orthogonally to the direction of elongation of theprisms/grooves. The physical width of each packet affects all colors oflight interacting with the packet, and the overall extracted light is anaverage effect of all the packets. Physical widths that are small tendto broaden the angular width of the out-coupled light, while physicalwidths that are large tend to narrow the out-coupled light angularwidth. However, the amount of angular broadening or narrowing that canbe achieved by physical width adjustment is somewhat limited becausephysical widths that are too small can lead to excessive light spreadingsuch that the diffractive surface features produce a high degree ofdistortion or scattering, and such that the light guide appears to bediffusive rather than diffractive.

Another technique for producing illumination that is more angularlydispersed (for better spatial uniformity at remote surfaces) is to use apattern of diffractive surface features oriented along differentin-plane directions, e.g., corresponding to different azimuthal anglesin the conoscopic plot of FIG. 5. The differently oriented diffractivefeatures are preferably also combined with corresponding light sourcesthat emit light generally along different in-plane directions tailoredfor maximum extraction efficiency with the corresponding diffractivefeatures. The combination of the variously oriented diffractive featuresand the variously oriented light sources can produce out-coupled lightemitted at a variety of azimuthal directions, resulting in illuminationthat is more angularly dispersed and more spatially uniform. In anexemplary embodiment, at least three distinct diffractive featureorientations can be used, corresponding to in-plane axes separated fromeach other by azimuthal angles of 120 degrees.

Differently oriented diffractive features can also be achieved throughthe use of continuously curved grooves or prisms, e.g., grooves orprisms that are circular, oval, or elliptical in shape (in plan view),or portions of such shapes, e.g., arcs, including series ofinterconnected arcs such as in sinusoidal or otherwise undulatingshapes. In that regard, embodiments disclosed herein that are describedas having linear diffractive surface features can alternatively employdiffractive features that are curved. Linear or curved diffractivesurface features, when combined with discrete light sources and/ornon-uniform reflective structures, can be used to produce visualfeatures in the form of bright or dark bands. Bands such as these arehighly undesirable in most extended source applications, but in somecases can be exploited to provide the lighting device with an aesthetic3-dimensional appearance that can enhance the appearance of thepattern(s) or indicia provided by the patterned printing discussedabove. Such patterned printing will now be discussed further, beginningwith FIG. 12.

In FIG. 12 a portion of a lighting device 1210 is shown schematically.The device 1210 includes a light guide 1212 having a first major surface1212 a, an opposed second major surface 1212 b, and at least one edge orside surface (not shown) at which one or more light sources (not shown)are disposed to inject light into the light guide 1212. The majorsurface 1212 a comprises diffractive surface features 1213 which, in thedepicted device portion, are coincident with the major surface 1212 a.The diffractive surface features 1213 are as described elsewhere herein,and may be: single-pitch over the entire major surface 1212 a, ormulti-pitch and arranged into packets; symmetric or asymmetric incross-sectional profile; straight or non-straight (including curved) inplan view; sized, in both pitch and height or depth, to extract orout-couple a significant amount of guided-mode light out of the lightguide by diffraction; and composed of a material having a givenrefractive index for visible wavelengths.

The lighting device 1210 also includes a discontinuous or patternedlight transmissive layer 1221 atop the major surface 1212 a. Thepatterned light transmissive layer 1221 is composed of a lighttransmissive medium and has portions 1221 a, 1221 b, 1221 c in opticalcontact with diffractive surface features 1213 in selected areas orregions 1251, but not in remaining regions 1250. The medium of layer1221 is a tangible material such as a polymer, adhesive, or gel, forexample, which has a given refractive index for visible wavelengths. Inthe remaining regions 1250 not covered by the layer 1221, thediffractive surface features 1213 are exposed to air or vacuum ofrefractive index 1.0.

By changing the refractive index of the medium the diffractive features1213 are exposed to, the patterned layer 1221 has the effect of changingthe extraction characteristics of the diffractive surface features 1213in the regions 1251, relative to the regions 1250 unaffected by thepatterned layer. Of relevance here are the refractive indices of threeelements: the diffractive surface features 1213, i.e., the prisms whichform the diffractive surface features; the optical medium or material oflayer 1221, which is in optical contact with the diffractive features1213 in regions 1251; and the optical medium (air, in this embodiment)in optical contact with the diffractive features 1213 in the remainingregions 1250. In general, the greater the difference in refractive indexbetween the diffractive surface features 1213 and the medium they are inoptical contact with, the greater or stronger the extraction efficiencyof the diffractive features. Conversely, the smaller the difference inrefractive index between the diffractive surface features 1213 and themedium they are in optical contact with, the smaller or weaker theextraction efficiency of the diffractive features.

Thus, if a large contrast in extracted light is desired between theregions 1250 and 1251, e.g. to produce highly visible indicia when thelighting device 1210 is turned on, then the material of layer 1221 maybe selected so that a second refractive index difference (“dn2”),between the refractive index of the layer 1221 and that of thediffractive features 1213, is as different as possible from a firstrefractive index difference (“dn1”), between the refractive index of airand that of the diffractive features 1213. For example, if thediffractive surface features 1213 are made of a polymer of refractiveindex 1.5, then dn1 is 0.5 (=1.5−1.0), and the material of layer 1221may be chosen so that dn2 is substantially smaller in magnitude than0.5, e.g., a refractive index for layer 1221 of 1.45 or 1.55 yields amagnitude of dn2=0.05, and a refractive index for layer 1221 of 1.49 or1.51 yields a magnitude of dn2=0.01. The magnitude of dn1 may be atleast 0.1, 0.2, 0.3, 0.4, or 0.5, and dn2 may be less than half that ofdn1, i.e., no more than 0.05, 0.1, 0.15, 0.2, or 0.25, respectively: dn2may also be close to zero in magnitude, e.g., less than 0.05, 0.04,0.03, 0.02, or 0.01. These values should not be considered limiting,however, and the refractive indices of the various layers may be chosenas desired to produce the desired amount of contrast between the regions1250 and 1251. Note that if the material of layer 1221 is selected tomatch the refractive index of the diffractive features 1213, such thatthe magnitude of dn2 is substantially zero, diffractive surface features1213 effectively disappear in the regions 1251 from an opticalstandpoint, and no light extraction occurs in those regions so long asthe upper exposed surface of layer 1221 is flat and smooth to promoteTIR of guided-mode light.

If on the other hand a small contrast in extracted light is desiredbetween the regions 1250 and 1251, e.g. to produce a subtle or barelyvisible indicia when the lighting device 1210 is turned on, then thematerial of layer 1221 may be selected so that the magnitude of dn2 isvery close to that of dn1. For example, if the diffractive surfacefeatures 1213 are made of a polymer of refractive index 1.5, then dn1 is0.5 (=1.5−1.0), and the material of layer 1221 may be chosen so that dn2is close in magnitude to 0.5, e.g., a refractive index for layer 1221 of1.2 yields dn2==0.3. In this example both the first and secondrefractive index differences are relatively large in magnitude, so thatlight extraction will be relatively strong in both regions 1250 and1251, but stronger in region 1250 due to the larger first refractiveindex difference of e.g. an air/polymer interface. Nanovoided materialshaving an ultra low index (ULI) of refraction are known that can comesomewhat close in index to air. See e.g. patent application publicationsWO 2010/120864 (Hao et al.) and WO 2011/088161 (Wolk et al.), whichdiscuss ULI materials in a range from about n≈1.15 to n≈1.35. See alsopatent application publications WO 2010/120422 (Kolb et al.), WO2010/120468 (Kolb et al.), WO 2012/054320 (Coggio et al.), and US2010/0208349 (Beer et al.). In still other embodiments, dn2 may bechosen to be substantially equal in magnitude but different in sign todn1 (or, if a large contrast is desired, different in sign and alsosubstantially different in magnitude). The refractive index for layer1221 may e.g. be 1.7 or more, or even about 2.0, whereupon dn1 and dn2may both have magnitudes of about 0.5, but opposite signs. The patternedlight transmissive layer 1221 may comprise any suitable material and maybe made using any suitable manufacturing process. Transparent polymermaterials, including in some cases adhesives including pressuresensitive adhesives, are preferred for ease of application, cost, anddurability, but not non-polymeric materials, such as inorganicmaterials, may also be used. In some cases it may be desirable for thelayer 1221 to have a high index of refraction, e.g., 1.6, 1.7, 1.8, 1.9,2.0, 2.1, or higher, while in other cases a low index of refraction maybe desired, e.g. 1.3, 1.2, or less for some ULI materials, while instill other cases an intermediate index of refraction, e.g. from about1.4 to 1.5, may be desired. The patterned layer 1221 may be highlytransmissive over the entire visible wavelength range, i.e., transparentand colorless, or it may be colored using suitable dye(s) and/orpigments so that transmission is high for some visible wavelengths butlow for other visible wavelengths. Selecting a colorless material forpatterned layer 1221 in FIG. 12 yields a pattern that is not readilyapparent to a user or ordinary observer when the lighting device 1210 isturned off, i.e., when the light sources disposed to inject light intothe light guide are turned off and little or no guided-mode lightpropagates within the light guide. Alternatively, the pattern or indiciacan be made to be noticeable and apparent to users or observers when thelighting device 1210 is turned off by selecting a colored, e.g.,pigmented and/or dyed, material for the patterned layer 1221.

The patterned light transmissive layer 1221 may be made using anysuitable processes now known or later developed, including coating,printing, laminating, depositing, dissolving, and/or etching. The layer1221 may be applied to the diffractive features 1213 in a selectivemanner, i.e., it may be applied to regions 1251 and not to regions 1250.Screen printing or ink-jet printing are two exemplary techniques forsuch selective application. The layer 1221 may alternatively be appliedto the diffractive features 1213 in a spatially uniform layer, and thenlater dissolved, etched, or otherwise removed selectively from theregions 1250. Keeping in mind that any of the light guides discussedherein may have a layered construction, the patterned layer 1221 may beapplied to a nanostructured major surface of a flexible polymer film,e.g. on a high volume film manufacturing line, and a piece of the coatedfilm may later be laminated to a light transmissive plate or othersubstrate to produce a light guide with diffraction extraction featuresand patterned printing as disclosed herein.

FIG. 13 depicts a portion of a lighting device 1310 that may be the sameas or similar to lighting device 1210, except that an additional layer1321 is included such that the diffractive features 1213 in regions 1250are no longer exposed to air, but instead are in optical contact withthe additional layer 1321. Thus, the diffractive surface features 1213in regions 1251 are in optical contact with the patterned layer 1221,but the diffractive surface features 1213 in the remaining regions 1250are in optical contact with the additional layer 1321. One advantage ofthis “buried” design over that of FIG. 12 is that all of the diffractivesurface features 1213, rather than just some of them, may be protectedfrom contamination or damage from dirt, dust, water, and other externalinfluences. Another advantage of FIG. 13 is that from a designstandpoint, if a small contrast in extracted light is desired betweenthe regions 1250 and 1251 to produce a subtle or barely visible indiciawhen the lighting device 1210 is turned on, then it is easier to nearlymatch the first refractive index difference (between the refractiveindex of the additional layer 1321 and that of the diffractive features1213) with the second refractive index difference (between therefractive index of the patterned layer 1221 and that of the diffractivefeatures 1213). That is because it is typically not difficult to selecta suitable material for the layer 1321 whose refractive index nearlymatches the refractive index for the patterned layer 1221. However, if alarge contrast in extracted light is desired between the regions 1250and 1251, materials for layers 1221, 1321 may alternatively be chosenwith very different refractive indices so that the first refractiveindex difference is much larger than, or much smaller than, the secondrefractive index difference.

The additional layer 1321 may comprise any suitable material and may bemade using any suitable manufacturing process. Transparent polymermaterials, including in some cases adhesives including pressuresensitive adhesives, are preferred for ease of application, cost, anddurability, but not non-polymeric materials, such as inorganicmaterials, may also be used. In some cases it may be desirable for thelayer 1321 to have a high index of refraction, e.g., 1.6, 1.7, orhigher, while in other cases a low index of refraction may be desired,e.g. 1.3, 1.2, or less for some ULI materials, while in still othercases an intermediate index of refraction, e.g. from about 1.4 to 1.5,may be desired. The additional layer 1321 may be highly transmissiveover the entire visible wavelength range, i.e., transparent andcolorless, or it may be colored using suitable dye(s) and/or pigments sothat transmission is high for some visible wavelengths but low for othervisible wavelengths.

Numerous possible visual effects for the pattern defined by layer 1221are possible by tailoring the refractive index relationships between thediffractive surface features 1213, the patterned layer 1221, and theadditional layer 1321, which refractive index relationships are in turncontrolled by appropriate materials selection for these elements. Withregard to the visibility of the pattern when the lighting device 1310 isturned on, the pattern may be made: bright with high contrast, byproviding strong extraction of guided-mode light in the regions 1251 andweak extraction in the regions 1250; bright with low contrast, byproviding strong extraction in both regions 1250 and 1251; dim with highcontrast, by providing weak extraction in the regions 1251 and strongextraction in the regions 1250; and dim with low contrast, by providingweak extraction in both regions 1250 and 1251. Furthermore, thevisibility of the pattern when the lighting device 1310 is turned offcan also be tailored as desired by appropriate selection of the color(transmission over the visible wavelength range) of the patterned layer1221 and the additional layer 1321. If both materials are composed ofclear transparent materials, or even materials that are both colored butof approximately the same color, then the pattern may have a lowvisibility when the lighting device 1310 is turned off. The visibilityof the pattern can be increased by selecting materials for the layers1221, 1321 that have substantially different visible light transmissionspectra, i.e., substantially different colors.

Similar to the patterned light transmissive layer 1221, the additionallayer 1321 may be made using any suitable processes now known or laterdeveloped. Typically, the layer 1321 may simply be coated atop theexposed surface of pattern-printed diffractive surface features. If thelight guide 1212 has a layered construction, the additional layer 1321(as well as the patterned layer 1221) may be applied to a nanostructuredmajor surface of a flexible polymer film, e.g. on a high volume filmmanufacturing line, and a piece of the coated film may later belaminated to a light transmissive plate or other substrate to produce alight guide with diffraction extraction features and patterned printingas disclosed herein.

The patterned layer 1221 may be composed of only one printed material,e.g. a particular single ink, or it may comprise multiple inks orsimilar patterned materials that may be printed or selectively coated asdesired, e.g. in overlapping or non-overlapping patterns, and in asingle layer or in multiple individual layers. For example, portions1221 a and 1221 b may be composed of a first ink of a first color orrefractive index, and portion 1221 c may be composed of a second ink ofa different color or refractive index. Alternatively, portions 1221 a,1221 b, and 1221 c may be composed of respective first, second, andthird inks of differing colors and/or refractive indices. Theseprinciples can be extended to any desired number of inks or patternedmaterials without limit.

In the discussion above we say that the printed materials or inks haverefractive indices that differ from that of each other and from that ofthe diffractive surface features or prisms. In some cases, therefractive index of these various layers may be less significant, or maybe of no significance (e.g. the refractive index of the various layersmay either be different or the same), while other material propertiesmay instead be significant. The other material properties may forexample be an absorptive characteristic, e.g., one material may havesubstantially no absorption over visible wavelengths, for a transparentand clear appearance, while another material (such as one of the printedinks) may selectively absorb certain visible wavelengths or colors, fora colored appearance of a first non-white color, and still anothermaterial (such as a different one of the printed inks) may selectivelyabsorb other visible wavelengths or colors, for a colored appearance ofa second non-white color. Absorption characteristics may also be outsidethe visible spectrum, e.g., printed materials or inks that absorb atultraviolet and/or infrared (including near-infrared) wavelengths, withor without absorption in the visible region, may be used, e.g. insecurity applications. The other material properties may alternativelybe a fluorescent characteristic, e.g., one material may emit nofluorescence or phosphorescence when exposed to short wavelengthelectromagnetic radiation (such as blue, violet, or ultraviolet light),while another material (such as one of the printed inks) may emitfluorescence or phosphorescence of a first color when exposed to theshort wavelength radiation, and still another material (such as adifferent one of the printed inks) may emit fluorescence orphosphorescence of a second color when exposed to the short wavelengthradiation. The fluorescent materials may be or comprise fluorescentdyes, phosphors, quantum dots, and the like.

Furthermore, in some cases a micro-spatial dot density can be used as adesign parameter that can substitute for, or enhance, refractive index.For example, a material having a given intrinsic refractive index can beprinted in a micro-spatial pattern or array of dots, analogous to grayscale newspaper printing, in order to provide a brightness of extractedlight that corresponds to that of a non-micro-patterned layer of a lowerrefractive index. The individual dots in the array are typically smallerthan can be discerned by an ordinary user, e.g. from about 2 to about200 micrometers in diameter in plan view. The areal density of the dotsdetermines the brightness in the printed region, and can be used todefine an effective refractive index, where an areal density of 100%(dots merged together to form a non-micro-patterned layer) correspondsto the intrinsic refractive index of the material, and an areal densityof 0% (dots so sparse that none exist) corresponds to the refractiveindex of air or other surrounding material. Micro-patterned dot arrayscan for example be used to replace one or more of portions 1221 a, 1221b, and 1221 c in FIG. 13.

To reiterate, the image or pattern produced by the patternedlight-transmissive layer may be made up of micro-spatial dots. Themicro-spatial dots may be provided in an array of sizes and/or densitiesthat are obtained by an analysis or breakdown of a pre-existing solidimage.

FIG. 14 depicts a portion of a lighting device 1410 that may be the sameas or similar to lighting device 1310, except that another layer 1421 isincluded. The layer 1421 may be a carrier film for the layers 1221,1321. Alternatively or in addition, the layer 1421 may be a protectivelayer such as a hard coat and may provide antiglare and/oranti-fingerprint layers, coatings, or elements as well. The layer 1421may be thin or thick, flexible or rigid, and may be made of suitablelight transmissive materials such as polymers or glasses.

The pattern provided by the patterned layer such as layer 1221 may be ofany desired shape, size, or configuration, as permitted by the printingtechnique used and the size of the output area of the light guide. Thepattern may be regular, irregular, random, or semi-random. The patternmay be large enough to be easily discernible to users of the lightingdevice, or so small that it, or individual elements of it, are notdiscernible to such users. The pattern may form indicia, e.g.,alphanumeric characters, symbols, shapes, marks, or the like. In somecases, the pattern may be or comprise a group or corporate logo. Such acase is shown in FIG. 15. In that figure, a schematic plan view is shownof a portion of a lighting device 1510 having a light guide with a majorsurface 1512 a. Diffractive surface features are provided over all ofthe major surface 1512 a shown in the figure, but the diffractivesurface features are not shown in the figure to reduce visual clutter.The diffractive surface features and light guide may be the same as orsimilar to any of the diffractive surface features or light guidesdiscussed herein. A patterned light transmissive layer, which may be thesame as or similar to layer 1221 described above, is present on themajor surface 1512 a, and in optical contact with the diffractivesurface features in the regions 1551, but not in the regions 1550. Theregions 1551 are in the form of indicia or logos. The size of theindividual logos may be large enough to be easily discernible to usersof the lighting device, or so small that they are not discernible tosuch users.

The disclosed lighting devices, which generally include an extendedlight guide and diffractive surface features disposed on at least onemajor surface of the light guide to extract guided-mode light, may alsobe made to include other design elements that work synergistically withthe diffractive surface features. One such design element is a patternedlow index subsurface layer within the light guide. The patternedsubsurface layer may be patterned in a way that is the same as, similarto, or different from the patterning of the patterned layer discussedabove, which is in optical contact with the diffractive surfacefeatures. But unlike the patterned layer discussed above, the subsurfacelayer is disposed beneath (although typically close to) the majorsurface of the light guide containing the diffractive surface features.The subsurface layer is thus disposed in an interior of the light guidebetween the opposed major surfaces thereof, and the light guide has anon-unitary construction. The subsurface layer functions to selectivelyblock some guided mode light from reaching the diffractive surfacefeatures. This is accomplished by tailoring the subsurface layer to havefirst layer portions characterized by a lower refractive index than thebulk of the light guide, such that some of the guided mode lightpropagating in the bulk of the light guide is reflected by totalinternal reflection (TIR) at the first portions and prevented fromreaching the diffractive surface features. The first layer portionsreside in first regions of the light guide but not second regionsthereof, the first and second regions being coplanar and in some casescomplementary. The first and second regions may define a pattern that isregular, irregular, random, semi-random, or of any desired design.

In some cases, the subsurface layer is partially continuous with respectto the first and second regions. For example, a nanovoided polymericmaterial may be present in the first layer portions (in the firstregions), and the subsurface layer may also include second layerportions in which the same nanovoided polymeric material is alsopresent, the second layer portions residing in the second regions. Thenanovoided polymeric material may then extend continuously from anygiven first layer portion to any and all second layer portions that areadjacent to such first layer portion. The nanovoided polymeric materialmay provide the first portions of the subsurface layer with a refractiveindex that is substantially lower than the bulk of the light guide. Forexample, the refractive index of the first portions at visiblewavelengths may be less than 1.4, or less than 1.3, or less than 1.2.The nanovoided polymeric material may have a void volume in a range fromabout 10 to about 60%, or from about 20 to about 60%, or from about 30to about 60%, or from about 40 to about 60%. The second layer portionsof the subsurface layer may be composed of the nanovoided polymericmaterial and an additional material.

The additional material may occupy at least a portion of the void volume(and in some cases may substantially completely fill the interconnectednanovoids such that little or no void volume remains), and preferablyhas the effect of changing the refractive index of the second layerportions by at least about 0.03, e.g., from about 0.03 to about 0.5,from about 0.05 to about 0.5, or from about 0.05 to about 0.25, relativeto the first layer portions in which the additional material is notsubstantially present. In some cases the additional material may be thesame material as a binder used to form the nanovoided polymericmaterial. Further information regarding suitable subsurface layershaving the continuous nanovoided polymeric material construction can befound in the following commonly assigned U.S. patent applications, inwhich the subsurface layer is referred to as a variable index lightextraction layer: U.S. application Ser. No. 61/446,740, “Front-LitReflective Display Device and Method of Front-Lighting ReflectiveDisplay”, filed Feb. 25, 2011; U.S. application Ser. No. 61/446,642,“Variable Index Light Extraction Layer and Method of Illuminating WithSame”, filed Feb. 25, 2011; U.S. application Ser. No. 61/446,712,“Illumination Article and Device for Front-Lighting ReflectiveScattering Element”, filed Feb. 25, 2011; and U.S. application Ser. No.61/485,881, “Back-Lit Transmissive Display Having Variable Index LightExtraction Layer”, filed May 13, 2011.

In some cases, the subsurface layer is discontinuous with respect to thefirst and second regions. For example, the first layer portions (in thefirst regions) may be printed with a first material of relatively lowrefractive index, and the second regions may be filled with a secondmaterial of relatively high refractive index, e.g., having a refractiveindex substantially matching, or exceeding, that of the bulk of thelight guide. Here, unlike the partially continuous subsurface layerdescribed above, the second material in the second regions may have nocommon structure or composition relative to the first material in thefirst regions, and the subsurface layer may consist essentially of thefirst layer portions.

Exemplary embodiments that incorporate such subsurface layers are shownschematically in FIGS. 16 and 17. In FIG. 16, a light guide 1612includes opposed first and second major surfaces 1612 a, 1612 b, anddiffractive surface features 1613 are formed on the first major surface1612. A patterned light transmissive layer 1621, comprising at leastportion 1621 a, is provided atop the major surface 1612 a. The lightguide 1612, diffractive surface features 1613, and patterned layer 1621may be the same as or similar to corresponding elements describedelsewhere herein. The diffractive surface features 1613 may be providedby a microreplicated optical film 1611 c having a prism layer cast andcured on a carrier film. A major portion or bulk of the light guide 1612may be provided by a plate or other relatively thick substrate 1611 a,to which the microreplicated optical film 1611 c is attached indirectlythrough a subsurface film 1611 b. In the embodiment of FIG. 16, thesubsurface film 1611 b includes a carrier film on which is disposed apatterned low index subsurface layer 1603. The subsurface layer 1603comprises first layer portions 1603 a in first regions 1640, and secondlayer portions 1603 b in second regions 1630. Adhesive layers (notshown) may also be provided between the microreplicated optical film1611 c and the subsurface film 1611 b, and between the subsurface film1611 b and the substrate 1611 a, for reliable and robust attachment withno significant air gaps. Such adhesive layers, and the second layerportions 1603 b, and the carrier films, and the prism layer allpreferably have relatively high refractive indices that match,substantially match, or exceed the refractive index of the substrate1611 a, such that these components support the propagation ofguided-mode light along the light guide 1612 between the surfaces 1612a, 1612 b.

The first layer portions 1603 a of the subsurface layer 1603 comprise asuitable nanovoided polymeric material having a first refractive indexthat is substantially lower than that of the other components of thelight guide 1612. The nanovoided polymeric material may be or compriseany of the ultra low index (ULI) materials discussed elsewhere herein.Preferably, substantially all of each first layer portion 1603 aincludes the nanovoided polymeric material. Further, the index ofrefraction is preferably relatively spatially uniform within each firstlayer portion 1603 a, e.g., the refractive index may change by no morethan ±0.02 across a continuous transverse plane for each layer portion.The refractive index of the first portions 1603 a may be less than 1.4,or less than 1.3, or less than 1.2. The nanovoided polymeric materialmay have a void volume in a range from about 10 to about 60%, or fromabout 20 to about 60%, or from about 30 to about 60%, or from about 40to about 60%.

The second layer portions 1603 b in the second regions 1630 comprise thesame nanovoided polymeric material used in the first layer portions 1603a, but the second portions 1603 b also include an additional material.The additional material, which may permeate some or substantially all ofthe void volume of the nanovoided material, causes the second portions1603 b to have a second refractive index that is different from thefirst refractive index by at least about 0.03, e.g., from about 0.03 toabout 0.5, from about 0.05 to about 0.5, or from about 0.05 to about0.25. The index of refraction is preferably relatively spatially uniformwithin each second layer portion 1603 b, e.g., the refractive index maychange by no more than ±0.02 across a continuous transverse plane foreach layer portion.

As a result of the lower refractive index in the first regions 1640,guided-mode light (sometimes also referred to as supercritical light)that encounters the first layer portions 1603 a is reflected by TIR backtowards the major surface 1612 b before it reaches the major surface1612 a with the diffractive surface features 1613. That is, the firstlayer portions 1603 a deflect or block some of the guided-mode lightfrom reaching and interacting with the diffractive surface features inthe first regions 1640. This is depicted in FIG. 16 by guided-mode lightray 1616 b. On the other hand, the substantial matching (or exceeding)of the refractive index of the second layer portions 1603 b with thoseof the polymers, carrier films, and substrate 1611 a, causes guided-modelight that encounters the second layer portions 1603 b to continuepropagating substantially undisturbed to the first major surface 1612 a,where at least some of the light is extracted or out-coupled into thesurrounding medium, as described in detail above, by the diffractivesurface features 1613. This is depicted in FIG. 16 by guided-mode lightray 1616 a. The subsurface layer 1603 thus selectively, in apattern-wise fashion, deflects some of the guided-mode light within thelight guide 1612 so that it does not interact with the diffractivesurface features 1613.

FIG. 16a shows a schematic cross section of an exemplary embodiment ofthe patterned low index subsurface layer 1603. The layer 1603 includesfirst layer portions in first regions 1640, the layer portions in bothsuch regions comprising a nanovoided polymeric material. In someembodiments, the nanovoided polymeric material comprises a plurality ofinterconnected nanovoids as described for example in WO 2010/120422(Kolb et al.) and WO 2010/120468 (Kolb et al.). The plurality ofinterconnected nanovoids is a network of nanovoids dispersed in a binderwherein at least some of the nanovoids are connected to one another viahollow tunnels or hollow tunnel-like passages. The nanovoids or pores insuch nanovoided polymeric material can extend to one or more surfaces ofthe material.

The subsurface layer 1603 also includes a second layer portion in asecond region 1630 disposed between first regions 1640. The secondregion comprises the nanovoided polymeric material and an additionalmaterial. This additional material may occupy at least a portion of thevoid volume of the nanovoided polymeric material. The dashed lines inFIG. 16a are used to indicate general location of the first and secondregions, however, these dashed lines are not meant to describe any sortof boundary between the regions.

In some embodiments, a seal layer is disposed on the patterned low indexsubsurface layer in order to minimize penetration of contaminants intothe latter. For example, a seal layer may be disposed on the patternedlow index subsurface layer such that it is in between the patterned lowindex subsurface layer and an adhesive layer. For another example, aseal layer may be disposed on the patterned low index subsurface layersuch that it is in between the patterned low index subsurface layer andthe substrate or other constituent layer of the lightguide, and the seallayer may have a refractive index that is approximately equal to orgreater than that of the substrate or other layer. Suitable seal layersare discussed in the commonly assigned U.S. patent applications citedabove.

In FIG. 17, a light guide 1712 includes opposed first and second majorsurfaces 1712 a, 1712 b, and diffractive surface features 1713 areformed on the first major surface 1712. A patterned light transmissivelayer 1721, comprising at least portion 1721 a, is provided atop themajor surface 1712 a. The light guide 1712, diffractive surface features1713, and patterned layer 1721 may be the same as or similar tocorresponding elements described elsewhere herein. The diffractivesurface features 1713 may be provided by a prism layer 1711 c which iscast-and-cured, microreplicated, embossed, etched, or otherwise formedon a high index resin layer 1711 b. The resin layer 1711 b may in turnbe applied to a plate or other relatively thick substrate 1711 a, whichmay comprise a major portion or bulk of the light guide 1712. However,before the resin layer 1711 b is applied to the substrate 1711 a andcured, a patterned low index subsurface layer 1703 is pattern-wiseapplied to the substrate 1711 a. The subsurface layer 1703 comprisesfirst layer portions 1703 a in first regions 1740, but the subsurfacelayer 1703 is either not applied to, or is applied to and later removedfrom, the substrate 1711 a in second regions 1730. Thus, at the time ofapplication of the resin layer 1711 b, the resin layer fills in thespaces in the second regions 1730. If desired, adhesive layers (notshown) and carrier films (not shown) may also be included in theconstruction, depending on the details of manufacture. Any such adhesivelayers and carrier films, as well as the resin layer 1711 b and theprism layer 1711 c, all preferably have relatively high refractiveindices that match, substantially match, or exceed the refractive indexof the substrate 1711 a, such that these components support thepropagation of guided-mode light along the light guide 1712 between thesurfaces 1712 a, 1712 b.

The first layer portions 1703 a of the subsurface layer 1703 arecomposed of a low index material having a first refractive index that issubstantially lower than that of the other components of the light guide1712. In some cases, the low index material may be or comprise ananovoided material such as those discussed in connection with FIGS. 16and 16 a, e.g., a ULI material. In other cases, the low index materialmay be an optical material that is not nanovoided, e.g., a UV curableresin comprising at least one fluorinated monomer, at least onefluorinated oligomer, at least one fluorinated polymer, or anycombination of such fluorinated materials. Preferably, the refractiveindex of the first portions 1703 a is less than 1.47, or less than 1.43,or less than 1.4, or less than 1.3, or less than 1.2.

The high index resin layer 1711 b may be composed of any suitablepolymer or other light-transmissive material having a suitably highrefractive index so that a substantial amount of guided-mode light canpropagate from the substrate 1711 a to the prism layer 1711 c.

As a result of the lower refractive index in the first regions 1740,guided-mode or supercritical light that encounters the first layerportions 1703 a is reflected by TIR back towards the major surface 1712b before it reaches the major surface 1712 a with the diffractivesurface features 1713. That is, the first layer portions 1703 a blocksome of the guided-mode light from reaching and interacting with thediffractive surface features in the first regions 1740. This is depictedin FIG. 17 by guided-mode light ray 1716 b. On the other hand, thesubstantial matching (or exceeding) of the refractive index of the resinlayer 1711 b with those of the polymers, carrier films, and substrate1711 a, causes guided-mode light that encounters the second regions 1730to continue propagating substantially undisturbed to the first majorsurface 1712 a, where at least some of the light is extracted orout-coupled into the surrounding medium, as described in detail above,by the diffractive surface features 1713. This is depicted in FIG. 17 byguided-mode light ray 1716 a.

The pattern provided by the patterned low index subsurface layer (e.g.,layers 1603, 1703) may be closely related, loosely related, or notrelated at all to the pattern provided by the patterned lighttransmissive layer (e.g., layer 1221 in FIGS. 12-14). In some cases, thesubsurface pattern may be a gradient pattern designed to deliver uniformlight or luminance to the diffractive surface features on the majorsurface of the light guide. In such cases, the printed pattern on thediffractive surface features can be any desired shape or image, and noregistration of any kind between the two patterns is needed. Typically,the two patterns would at least partially overlap in such cases. Inother cases, the subsurface pattern may be in the form of a specificimage (e.g. indicia), whether a solid image print or dithered print inthe shape of an image. In these cases, the printed pattern on thediffractive surface features may be registered with the subsurfacepattern, but such registration is not required. For example, in somecases, for aesthetic or artistic purposes, distinctly different imageswith no particular alignment or registration can be provided by the twopatterns. Such patterns, which would typically be at least partiallyoverlapping, can be used to create interesting levels of contrast in theillumination scheme to provide a unique appearance for the lightingdevice. However, in some cases, alignment or registration of the twopatterns can be used to amplify the visual effect or contrast offoreground and background areas of the patterns, e.g., by selectivelydelivering more light to printed areas of the diffractive surfacefeatures and blocking light from reaching non-printed areas of thediffractive surface features, or vice versa. In that regard, thepatterns can be made to be spatially complementary, and registered toeach other such that the subsurface pattern delivers light substantiallyonly to non-printed regions of the diffractive surface features, whichmay also result in a contrast enhancement of the image. In still othercases, the two patterns may be the same or similar to each other, butoffset in registration by a controlled amount to provide a shadowingeffect, such as the shadowing effect used for displayed text commonlyused in computer presentation software.

In addition to being useful as luminaires for illuminating work spaces,living areas, and the like, the lighting devices disclosed herein mayalternatively be useful as illuminated security features, wherein thepattern(s) provided by the printed layer(s) provide indicia that may becovert and/or overt in nature as desired. In some security applications,the device may be incorporated into or applied to a product, package, ordocument, e.g. as an indicator of authenticity since the visual featuresare difficult to copy or counterfeit. Such security applications mayinclude: cards of various types including identification cards, socialsecurity cards, health cards, insurance cards, business cards,membership cards, voter registration cards, phone cards, stored valuecards, gift cards, border crossing cards, immigration cards, andfinancial transaction cards (including credit cards and debit cards);badges; passports; drivers licenses; vehicle license plates; gun permitsand other permits; event passes; advertising promotions; product tagsincluding hang-tags; product packaging; labels; charts; maps; and othersecurity articles and documents.

In some cases, such as in a card, one or more miniature light sourcessuch as LEDs may be included in the construction at or near an edge ofthe card to provide the guided-mode light. In other cases, such as inthe case of a passport or other security document, but also in the caseof cards, light sources may not be included in the article itself, butthe article may be configured for use with a reader or similar testingdevice that contains one or more suitable light sources adapted tocouple to an edge (or other surface) of the card or document to injectlight into the light-transmissive layer or layers that make up the lightguide, or the article may be configured for use with natural lightsources. The light guide may be relatively thick and rigid, as in thecase of a clear light-transmissive financial transaction card, orrelatively thinner and flexible, as in the case of a polymer sheet foruse in a passport, for example.

Example 1

A lighting device suitable for use as a luminaire was made andevaluated. The device incorporated a circular-shaped light guide withdiffractive surface features in the form of a spiral pattern, thediffractive surface features arranged into repeating patterns of sixpacket types with different groove or prism pitches. A mounting ring wasused to position thirty-six equally spaced LEDs around the curved sidesurface of the light guide to inject light into the light guide. Indiciain the shape of a United States map was formed by patterned printing onthe diffractive surface features. Further details of construction willnow be given.

A precision diamond turning machine was used to cut a spiral-shapedgroove pattern, which became the diffractive surface features in thelighting device after replication, into the copper surface of acylindrical tool. The diamond was shaped so that the grooves had asawtooth (asymmetric) profile in cross section similar to FIG. 6, with aheight-to-pitch ratio (see FIG. 6) of about 1:1. During cutting, thegroove pitch of the spiral was cycled between six specific values (315nm, 345 nm, 375 nm, 410 nm, 445 nm, and 485 nm) to produce groovepackets which formed nested annular regions that bordered each other butdid not overlap with each other. Each annular region was a groove packetof constant pitch, and each set of six adjacent annular regions formed arepeating group or set of groove packets. The spiral pattern had anoverall diameter of about 8 inches (about 20 centimeters). The radialdimensions or widths of the annular regions were selected so that theaggregate area for all of the six pitch values was the same. That is,the area of the entire grooved pattern was about 314 cm² (πr², wherer≈10 cm), and the aggregate area for grooves having the 315 nm pitch wasabout 314/6≈52 cm², and the aggregate areas for grooves having each ofthe other five pitches was also about 52 cm². The annular regions wererelatively narrow as measured radially, the maximum such dimension beingabout 150 micrometers.

The grooved surface of the resulting copper tool was then replicated ina thin flexible light-transmissive film (see e.g. layers 1111 b and 1111c in FIG. 11) using a cast-and-cure technique. This was done by coatingthe grooved surface of the copper tool with an organic phosphonic acidrelease layer (commonly known to those skilled in the art), and castingan acrylate resin composition against the coated precision tool using atransparent polyethylene terephthalate (PET) support film having athickness of about 5 mils (about 125 micrometers). The acrylate resincomposition included acrylate monomers (75% by weight PHOTOMER 6210available from Cognis and 25% by weight 1,6-hexanedioldiacrylateavailable from Aldrich Chemical Co.) and a photoinitiator (1% by weightDarocur 1173, Ciba Specialty Chemicals). The resin composition was thencured using ultraviolet light. This resulted in a microreplicatedoptical film about 125 microns thick and having diffractive surfacefeatures in the form of a negative or inverted version (negativereplica) of the spiral-shaped groove pattern from the precision coppertool. The refractive index of the PET support film was about 1.49 andthe refractive index of the cured acrylate resin was about 1.5. Themicroreplicated optical film had a transparent appearance when viewed atan angle normal to the surface of the film, with a slightly blue hue.Objects could be viewed through the film with low distortion.

The microreplicated film was then pattern printed in the shape of aUnited States map. This was done by first obtaining a sheet of linedoptically clear pressure sensitive adhesive (PSA), sold as Vikuiti™ OCA8171 by 3M Company. This product is sold as a 1 mil (about 25micrometer) thick PSA layer sandwiched between two 2 mil (50 micrometer)thick release liners. Ordinarily, when the product is used to bond twoarticles together, the release liners are removed and discarded, so thatonly the 1 mil thick PSA layer is present between the bonded articles.In the present case, we removed a first one of the release liners fromthe sheet, and pressed the exposed adhesive layer (still attached on theother side to the original second release liner) against a 2 mil (50micrometer) PET film. We then removed the second release liner, thusexposing the other surface of the PSA layer. A UV-curable clear ink(product code UV OP1005 GP Varnish, available from Nazdar Company,Shawnee, Kans.) was then printed onto the exposed surface of the PSAlayer using an indirect gravure printing process. Printing was doneusing a flexographic tool having a pattern of a United States map. Theline speed during printing was about 10 meters/minute using an aniloxroll, the anilox roll having an approximate volume of 5 billion cubicmicrons/square inch, and rated to give a wet coating thickness of about4 microns. After printing, some portions of the formerly exposed surfaceof the PSA layer were coated with the ink, and remaining portions wereleft uncoated and exposed to the air. The uncoated portions correspondedto foreground areas of the map image, and the coated portionscorresponded to the background areas of the map image. The ink was thencured (cross-linked) using ultraviolet light from a mercury vapor lamp(“H” bulb) to form a clear, non-adhesive amorphous glass-like layer,approximately 4 microns thick, on selected portions of the surface ofthe PSA layer corresponding to the background areas of the map image.

The resulting printed sheet was then pressed against the surface of themicroreplicated optical film containing the diffractive surface featuresto form a patterned laminate. Portions of the PSA layer that were notcoated with the cured ink, corresponding to the background areas of themap image, flowed into and filled the spaces between the diffractivesurface features, so that optical contact was made between the PSAlayer, which had a refractive index of about 1.475, and the diffractivesurface features, which had a refractive index of about 1.5. Portions ofthe PSA layer that were coated with the cured ink, corresponding to theforeground areas of the map image, did not flow into or make opticalcontact with the diffractive surface features due to the presence of theglass-like cured ink. In those areas, a very thin air pocket or layerremained between the cured ink and the diffractive surface features suchthat the diffractive surface features were in optical contact with air.

Excess material around the spiral diffractive feature area was cut awayso that the patterned laminate was circular in shape. The patternedlaminate was directly attached to one major surface of a clear,light-transmissive circular acrylic plate of thickness 3 mm, the platealso having a diameter of about 20 cm. Attachment was carried out suchthat the 2 mil PET support film of the printed adhesive became theoutermost layer of the construction, with the diffractive surfacefeatures (some in optical contact with the optically clear PSA, othersin optical contact with the thin air layer) disposed between the PETsupport film and the plate. The combination of the plate and thepatterned laminate resulted in a light guide with diffractive surfacefeatures on (only) one major surface thereof for light extraction, andwith patterned printing forming indicia (a United States map image), thelight guide having a diameter of about 20 cm and a thickness of about 3mm.

A string of 36 nominally identical LEDs (product code NCSL119T-H1 fromNichia), each LED emitting white light (“warm white”) in a divergentdistribution, was used for light injection into the light guide. TheLEDs were mounted in a ring-shaped bezel so that they were equallyspaced in 10 degree increments around the circular side surface of thelight guide, each LED pointed towards the center of the light guide anddisposed immediately adjacent the side surface to directly inject lightinto the light guide. For improved efficiency, strips of highreflectivity mirror film (3M™ Vikuiti™ ESR) were laminated on the insidesurface of the mounting ring between every two neighboring LEDs, themirror film strips also being immediately adjacent to the circular sidesurface of the light guide.

The lighting device so constructed was connected to a power supply andplaced sideways on a table in a laboratory setting, with the symmetry oroptical axis of the light guide parallel to the floor. FIG. 18a is aphotograph of the lighting device with the power supply turned off andambient room lights turned on. The viewing direction for this photographwas approximately head-on, i.e., along the optical axis of the lightguide. Note that objects across the room can be seen through the lightguide with little or no significant distortion. Wires used to connectthe lighting device to the power supply can also be seen through thelight guide. In this “off” state, the light guide had a slightly bluishhue similar to that of the microreplicated film by itself. FIG. 18b is aphotograph of the same lighting device from the same viewing directionas FIG. 18a , but with the power supply (and thus all 36 LEDs) turned onand the ambient room lights turned off. The U.S. map of the patternedprinting is clearly visible, and the contrast between printed regionsand remaining regions is high. Variable color hues could also be seen atdifferent areas of the light guide, the colors not being visible in thegrayscale photograph of FIG. 18b . Bright bands can also be seen overthe output area of the light guide, superimposed on the printed pattern,one band for each of the 36 energized light sources, and these bands areplainly visible in FIG. 18b . The bands are all relatively straight(radial) with little or no curvature from the viewing geometry of FIG.18 b. When observed at other viewing directions, the printed patternchanged appearance in the same way that any flat, 2-dimensional imagechanges appearance from different perspectives. In contrast, the bandschanged appearance as if they formed a 3-dimensional structure with eachband being a curved arch lying in a radial plane perpendicular to theplane of the light guide. Thus, at oblique viewing angles, some of thebands changed in apparent shape from straight to curved. This can beseen in FIG. 18c , which is a photograph of the same lighting device butat an oblique viewing angle, and taken from the opposite side of thelighting device. Variable color hues could also be seen across the lightguide at virtually any viewing direction.

Example 2

Another lighting device suitable for use as a luminaire was made andevaluated. The device incorporated a circular-shaped light guide withdiffractive surface features. The diffractive surface features filled 36triangle-shaped areas which were uniformly sized and tiled tosubstantially fill the circular area of the light guide. The diffractivesurface features in each of the triangle-shaped areas were straight andparallel to each other, and of a single pitch, although three differenttriangle types of three different pitches were used. A mounting ring wasused to position thirty-six equally spaced LEDs around the curved sidesurface of the light guide to inject light into the light guide. Indiciain the shape of a United States map was formed by patterned printing onthe diffractive surface features. Further details of construction willnow be given.

A precision diamond turning machine was used to cut linear grooves intothe copper surface of a cylindrical tool. The diamond was shaped so thatthe grooves had a sawtooth (asymmetric) profile in cross section similarto FIG. 6, with a height-to-pitch ratio (see FIG. 6) of about 1:1.During cutting, the groove pitch was maintained at a constant value ofabout 310 nm to produce a first single-pitch one-dimensional diffractiongrating tool. This procedure was then repeated in another copper surfaceusing a different groove pitch, the pitch now being maintained at aconstant value of about 345 nm to produce a second single-pitchone-dimensional diffractive grating tool. The procedure was repeated athird time in still another copper surface using a third groove pitch,the third pitch being maintained at a constant value of about 410 nm toproduce a third single-pitch one-dimensional diffractive grating tool.

The grooved surfaces of the resulting three copper tools were thenreplicated in three corresponding thin flexible light-transmissive films(see e.g. layers 1111 b and 1111 c in FIG. 11) using a cast-and-curetechnique. This was done by coating the grooved surface of each coppertool with an organic phosphonic acid release layer (commonly known tothose skilled in the art), and casting an acrylate resin compositionagainst the coated precision tool using a transparent polyethyleneterephthalate (PET) support film having a thickness of about 5 mils(about 125 micrometers). The acrylate resin composition includedacrylate monomers (75% by weight PHOTOMER 6210 available from Cognis and25% by weight 1,6-hexanedioldiacrylate available from Aldrich ChemicalCo.) and a photoinitiator (1% by weight Darocur 1173, Ciba SpecialtyChemicals). The resin composition was then cured using ultravioletlight. This resulted in three microreplicated optical films, each about125 microns thick and having diffractive surface features in the form ofnegative or inverted versions (negative replicas) of the linear groovepattern from the first, second, and third precision copper tools,respectively. The refractive index of the PET support film was about1.49 and the refractive index of the cured acrylate resin was about 1.5.Each of the microreplicated optical films had a transparent appearancewhen viewed at an angle normal to the surface of the film, with aslightly blue hue. Objects could be viewed through each film with lowdistortion.

The microreplicated optical films were then physically cut intotriangle-shaped pieces, twelve such pieces obtained from each of thefirst, second, and third optical films. The pieces were substantiallyidentically shaped into isosceles triangles with two long edges and oneshort edge, the long edges each being about 100 mm in length and theshort edge being about 17 mm in length. The pieces were all cut fromtheir respective optical films such that the diffractive surfacefeatures completely filled one major surface of the triangle piece, andthe individual grooves or prisms of the diffractive surface featureswere all parallel to the short edge of the triangle shape.

All thirty-six of the triangle-shaped pieces of optical film were thendirectly attached to one major surface of a clear, light-transmissivecircular acrylic plate of thickness 3 mm, the plate having a diameter ofabout 20 cm. For the attachment, the triangle-shaped pieces were laidnext to each other in a tiled arrangement with the long edges ofadjacent pieces abutting each other, and with the short edges of thepieces forming a thirty-six sided shape approximating a circle andsubstantially coinciding with the circular outer side surface of theacrylic plate. The film pieces were also arranged in a repeatingsequential 1,2,3,1,2,3, . . . fashion such that any given piece from thefirst film abutted a piece from the second film along one long edge andabutted a piece from the third film along the other long edge.Attachment of the pieces to the plate was accomplished using a 1 mil(approximately 25 micrometer) thick optically clear pressure sensitiveadhesive (Vikuiti™ OCA 8171 from 3M Company), with the microreplicatedsurface of each film piece facing away from the plate and exposed toair, and with substantially no air gaps between each film piece and theplate. The combination of the plate and the thirty-six film piecesresulted in a light guide with diffractive surface features on (only)one major surface thereof for light extraction, the light guide having adiameter of about 20 cm and a thickness of about 3 mm.

A sheet of lined optically clear pressure sensitive adhesive (PSA) wasthen obtained and printed with a curable ink in the pattern of a UnitedStates map substantially as described in Example 1. After the ink wascured, the resulting printed sheet was joined to the light guide bypressing the printed sheet against the surface of the microreplicatedoptical film pieces containing the diffractive surface features.Portions of the PSA layer that were not coated with the cured ink,corresponding to the background areas of the map image, flowed into andfilled the spaces between the diffractive surface features, so thatoptical contact was made between the PSA layer, which had a refractiveindex of about 1.475, and the diffractive surface features, which had arefractive index of about 1.5. Portions of the PSA layer that werecoated with the cured ink, corresponding to the foreground areas of themap image, did not flow into or make optical contact with thediffractive surface features due to the presence of the glass-like curedink. In those areas, a very thin air pocket or layer remained betweenthe cured ink and the diffractive surface features such that thediffractive surface features were in optical contact with air. Thecombination of the light guide (the plate and the optical film pieces)and the printed sheet resulted in a light guide with diffractive surfacefeatures on (only) one major surface thereof for light extraction, andwith patterned printing forming indicia (a United States map image), thelight guide having a diameter of about 20 cm and a thickness of about 3mm.

A string of 36 nominally identical LEDs (product code NCSL119T-H1 fromNichia), each LED emitting white light (“warm white”) in a divergentdistribution, was used for light injection into the light guide. TheLEDs were mounted in a ring-shaped bezel so that they were equallyspaced in 10 degree increments around the circular side surface of thelight guide, each LED pointed towards the center of the light guide anddisposed immediately adjacent the side surface to directly inject lightinto the light guide. For improved efficiency, strips of highreflectivity mirror film (3™ Vikuiti™ ESR) were laminated on the insidesurface of the mounting ring between every two neighboring LEDs, themirror film strips also being immediately adjacent to the circular sidesurface of the light guide.

The lighting device so constructed was connected to a power supply andplaced sideways on a table in a laboratory setting in the same way as inExample 1. With the power supply turned off and in ambient room light,objects across the room could be seen through the light guide withlittle or no significant distortion. Furthermore, in this “off” state,the light guide had a slightly bluish hue similar to that of themicroreplicated film by itself, and the printed image of the U.S. mapcould not be easily perceived. FIG. 19 is a photograph of the lightingdevice of this Example 2 with the power supply (and thus all 36 LEDs)turned on and the ambient room lights turned off. The U.S. map of thepatterned printing is clearly visible, and the contrast between printedregions and remaining regions is high. Variable color hues could also beseen at different areas of the light guide, with differenttriangle-shaped areas having different colors (particularly in theforeground areas of the map image) which are discernible in FIG. 19 eventhough the colors themselves are not visible due to the grayscale formatof the photograph. Straight radial border features, which are relativelybright and caused by light scattering at the edges of the individualtriangle-shaped pieces, can also be seen in the photograph. In additionto these bright border features, additional fainter radial bands canalso be seen in some places over the output area of the light guide,superimposed on the printed pattern, these fainter bands beingassociated with particular ones of the energized light sources. Thefainter bands are all relatively straight (radial) with little or nocurvature from the viewing geometry of FIG. 19, but their shape changedas a function of viewing angle in a 3-dimensional fashion in the sameway as the bright bands of Example 1 changed. Variable color hues couldalso be seen across the light guide at virtually any viewing direction.

Example 3

Another lighting device suitable for use as a luminaire was made andevaluated. The device incorporated a rectangular-shaped light guide withdiffractive surface features. The diffractive surface features were aportion of the spiral-shaped groove pattern described in Example 1, theportion taken from a central rectangular region of the spiral pattern. Alight source module was mounted along one of the short edges of therectangular light guide, the light source module containing one row ofeighteen equally spaced individual, discrete light sources, the lightsources being nominally identical LEDs each emitting white light in adivergent distribution. The light guide also incorporated a patternedlow index subsurface layer in the form of a random gradient dot pattern.The light guide was also capable of incorporating patterned printing,such as the United States map indicia or other desired indicia, incontact with the diffractive surface features. Further details ofconstruction will now be given.

The following ingredients were combined in a 1-liter wide-mouth amberbottle: 5.70 g of an aliphatic urethane oligomer (product code CN 9893from Sartomer Company, Exton, Pa.), and 22.40 g of pentaerythritoltriacrylate (product code SR 444, also from Sartomer Company). Thebottle was capped and shaken for 2 hours to dissolve the CN9893 toproduce a clear batch. This solution, referred to as a resin premix, wascombined with 482.84 g of silane treated (product code Silquest™ A-174from Momentive Performance Materials, Friendly, W. Va.) colloidal silica(product code NALCO 2327 from Nalco Chemical Co., Naperville, Ill.) in a2000 mL poly bottle. These components were mixed by transferring thebatch back and forth between the two bottles, ending with the batch inthe 2000 mL poly bottle. To this bottle was added, 5.84 g of a firstphotoinitiator (product code IRGACURE™ 184 from Ciba Specialty ChemicalsCorp., Tarrytown, N.Y.) and 1.12 g of a second photoinitiator (productcode IRGACURE™ 819, also from Ciba Specialty Chemicals Corp.). Thesolution was shaken for 30 minutes to dissolve the photoinitiators. Theresulting batch was a translucent, low-viscosity dispersion. The batchwas then diluted to about 17.7% solids by weight with a 50/50 blendethyl acetate and propylene glycol methyl ether (available from DowChemical as DOWANOL PM), to yield a coating formulation.

The coating formulation was coated onto a 50 micron thick PET film(MELINEX 617, available from DuPont) using a slot die at a line speed of3.1 meters/minute. The wet coating thickness was approximately 8.1microns. In an inert chamber (<50 ppm O₂), the wet coating was partiallycured in-line at the same line speed with UV radiation at 395 nm anddose of 850 mJ/cm². The UV radiation was provided by UV-LEDs availablefrom Cree, Inc. The partially cured coating sample was then dried at 70°C. in a 9 meter oven, and under a nitrogen-purged atmosphere, finallycured with a 236 Watt/cm² Fusion H bulb (available from Fusion UVSystems, Inc.). The resulting nanovoided polymeric layer had a thicknessof 1.3 microns. The transmission was 96.4%, the haze was 1.33%, and theclarity was 99.7%, as measured using a BYK gardner Haze Gard Plus(Columbia, Md.) instrument. The refractive index of the nanovoided layerwas between 1.20 and 1.22 as measured at 589 nm using a Metricon PrismCoupler (Metricon Corporation, Pennington, N.J.).

The nanovoided polymeric layer, still disposed on the 50 micron PETcarrier film, was printed with a UV curable clear ink (UV OP1005 GPVarnish from Nazdar, Shawnee, Kans.) using an indirect gravure printingprocess. A flexographic tool was fabricated to have a random 100 microngradient dot pattern, the density of the dots varying in an in-plane xdirection and being relatively constant in an orthogonal in-planey-direction. The gradient pattern was similar to that shown in thephotograph of FIG. 20. A gravure roll (pyramidal and 9 cubic microns persquare micron) was rated to give a wet coating of approximately 9.65microns. The printing was done at 10 meters per minute with highintensity UV curing under a nitrogen-purged atmosphere with a 236Watt/cm² Fusion H bulb (available from Fusion UV Systems, Inc.) afterthe printing. The resulting printed layer was made up of: first regionshaving the nanovoided polymeric material, the first regions having afirst refractive index; and second regions having the same nanovoidedpolymeric material but wherein the nanovoids were filled or partiallyfiled with the cured clear ink, the second regions having a secondrefractive index greater than that of the first regions. The opticalfilm consisting of this dot-printed nanovoided layer atop the 50 micronPET carrier film was substantially transparent, and objects could beseen with little distortion when looking through the film. Opticalproperties of this optical film, before being incorporated into thelight guide, were measured using the BYK Gardner Haze Gard Plusinstrument. Measurements made on one side or end of the film, at whichthe random gradient dot pattern had a low density, were: 96.6%transmission; 3.56% haze; and 95.6% clarity. Measurements made on theopposite side or end of the film, at which the random gradient dotpattern had a high density, were: 95.8% transmission; 6.82% haze; and89.9% clarity. Note that the transmission measurements reported here arenot corrected for Fresnel reflections at the outer surfaces of the film.The refractive index of the cured ink was measured to be approximately1.525 as measured on a flat cured sample using a Metricon prism coupler(wavelength of light used to measure the refractive index was 589 nm).

A lighting device was then made using this dot-printed optical filmtogether with a rectangular acrylic plate, a rectangular piece orportion of the microreplicated optical film (having the spiralmulti-pitch diffractive surface features) described above in Example 1,and a linear array of discrete light sources. The dot-printed nanovoidedlayer of the dot-printed optical film was used as a patterned low indexsubsurface layer to spatially control the interaction of guided-modelight with curved diffractive surface features. A rectangular section orpiece was cut out of a microreplicated optical film as described inExample 1, the center of the rectangular piece substantially coincidingwith the center of the spiral groove pattern. The rectangular piece hada major in-plane dimension (length) of about 6 inches (about 150 mm) anda minor in-plane dimension (width) of about 4 inches (about 100 mm). Arectangular acrylic (PMMA) plate was obtained having a major in-planedimension (length) of about 6 inches (about 150 mm), a minor in-planedimension (width) of about 4 inches (about 100 mm), and a thickness of 3mm. A piece of the dot-printed optical film described above was attachedto one of the major surfaces of the acrylic plate using a pressuresensitive adhesive, 3M Optically clear adhesive 8171. The rectangularpiece of the microreplicated optical film was attached to the oppositeside of the dot-printed optical film using an additional layer of 3MOptically Clear Adhesive 8171, such that the microreplicated surface(diffractive surface features) faced away from the acrylic plate and wasexposed to air, and such that the dot-printed nanovoided layer wasburied or sandwiched between the microreplicated film and the acrylicplate with substantially no air gaps between the film pieces and theplate, the dot-printed nanovoided layer thus forming a patterned lowindex subsurface layer. The combination of the films and the plateresulted in a light guide with diffractive surface features on (only)one major surface thereof, the light guide having in-plane dimensions ofabout 6 inches and 4 inches (about 150 mm and 100 mm) and a thickness ofabout 3 mm.

The light guide so constructed was then placed into an illumination testfixture which contained a light source module having eighteen equallyspaced discrete light sources, the light sources being nominallyidentical LEDs (product code NCSL119T-H1 from Nichia), each LED emittingwhite light (“warm white”) in a divergent distribution. The light sourcemodule was mounted along the short side of the light guide. The lightsources were energized with a power supply and photographs were taken ofthe lighting device from various viewing geometries. A strip of blackelectrical tape was placed on one side of the LED array to block straylight, emitted in sideways directions from the LEDs, from reaching thecamera. A photograph of the lighting device when viewed from a positionsubstantially perpendicular to the face of the light guide is shown inFIG. 21a . In this view, the light sources are on the right side of thisfigure. A photograph of the same lighting device when viewed at anoblique angle to the plane of the light guide is shown in FIG. 21b . Asa result of the gradient patterned low index subsurface layer, thelighting device exhibited a uniform-appearing luminance distributionwhen looking at the light guide, and also provided, on a diffusivesurface located 1 meter from the light guide approximately along theoptical axis, illumination that was substantially uniform in color.Bright bands associated with the discrete light sources can be clearlyseen in each of the viewing geometries, and the bands were observed tochange in shape and curvature with viewing geometry. Variable color huescould also be seen at different areas of the light guide, but the colorsare not visible in the grayscale photograph of the figures.

Although the lighting device of this Example 3 did not include patternedprinting of a light transmissive material on the diffractive surfacefeatures of the light guide, such patterned printing, whether in theform of the U.S. map of Examples 1 and 2 or in the form of other indiciaas desired, may be readily included in the construction to achieveresults in conformity with those of Examples 1 and 2.

The teachings of this application can be used in combination with theteachings of any or all of the following commonly assigned patentapplication publications, which are incorporated herein by reference: US2014/0043846 (Yang et al.); US 2014/0043847 (Yang et al.); and US2014/0043856 (Thompson et al.).

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. The readershould assume that features of one disclosed embodiment can also beapplied to all other disclosed embodiments unless otherwise indicated.It should also be understood that all U.S. patents, patent applicationpublications, and other patent and non-patent documents referred toherein are incorporated by reference, to the extent they do notcontradict the foregoing disclosure.

The invention claimed is:
 1. A lighting device, comprising: a light guide having a first major surface, the first major surface having first and second diffractive surface features formed in first and second portions thereof respectively, at least one of the first and second diffractive surface features adapted to couple guided-mode light out of the light guide; and a patterned light transmissive layer in optical contact with the second diffractive surface features but not the first diffractive surface features, the patterned layer comprising a second light transmissive medium; a first light transmissive medium in optical contact with the first diffractive surface features but not the second diffractive surface features; wherein the first and second light transmissive media have different first and second refractive indices respectively at a visible wavelength; and wherein the light guide is non-flat.
 2. The device of claim 1, wherein the light guide is simply curved.
 3. The device of claim 1, wherein the light guide is complex curved.
 4. The device of claim 1, wherein the first and second refractive indices differ by at least 0.05.
 5. The device of claim 1, wherein the first and second portions of the first major surface define indicia.
 6. The device of claim 1, further comprising: one or more light sources disposed proximate the light guide to provide the guided-mode light in the light guide.
 7. The device of claim 1, wherein the first light transmissive medium is air.
 8. The device of claim 1, wherein the second light transmissive medium is an adhesive.
 9. The device of claim 1, wherein the first and second light transmissive media are both polymer compositions.
 10. The device of claim 1, wherein the first and second light transmissive media are both substantially transparent and colorless.
 11. The device of claim 1, wherein the first major surface comprises a first group of diffractive surface features having a first uniform pitch and a second group of diffractive surface features having a second uniform pitch.
 12. The device of claim 1, wherein the first major surface comprises groups of diffractive surface features having differing pitches to form pitch-related indicia.
 13. The device of claim 12, wherein the first and second portions of the first major surface define second indicia.
 14. The device of claim 13, wherein the pitch-related indicia is in registration with the second indicia.
 15. The device of claim 13, wherein the pitch-related indicia is not in registration with the second indicia.
 16. A lighting device, comprising: a light guide having a first major surface, the first major surface having first and second diffractive surface features formed in first and second portions thereof respectively, at least one of the first and second diffractive surface features adapted to couple guided-mode light out of the light guide; and a patterned light transmissive layer in optical contact with the second diffractive surface features but not the first diffractive surface features, the patterned layer comprising a second light transmissive medium; a first light transmissive medium in optical contact with the first diffractive surface features but not the second diffractive surface features; wherein the first and second light transmissive media have different first and second refractive indices respectively at a visible wavelength; and wherein the first major surface comprises groups of diffractive surface features having differing pitches to form pitch-related indicia.
 17. The device of claim 16, wherein the first and second portions of the first major surface define second indicia.
 18. The device of claim 17, wherein the pitch-related indicia is in registration with the second indicia.
 19. The device of claim 17, wherein the pitch-related indicia is not in registration with the second indicia. 